Who is your carrier? Both people might have an iPhone, but if one has Verizon and the other has AT&T the experience is different because both are connected to completely different technologies and totally separate networks. AT&T and T-Mobile use GSM (Global System for Mobile) technology, the technology that is used in most of the rest of the world. But Verizon and Sprint use CDMA (Code Division Multiple Access) technology. These technologies are so different that a handset that is made only for one technology won’t work on the other. This is why you can’t take your Verizon handset to most of the rest of the world when you travel.
Who’s on the nearest tower? I’ve often been driving with somebody and hear them be glad to see an upcoming cell tower because they assume this means they’ll get better coverage. But you can’t assume this because not every carrier is on every cell tower. There are a large number of cell towers in the country. Some of these are owned by the wireless carriers but many are leased. The cellular companies look at available towers and then cobble together the combination of towers that make the most effective and cost-efficient network for them.
This task has gotten hard for the carriers because of the fact that cellphones now carry data. The original cell tower network with all of the giant towers was created back when cellphones only carried voice. But now that the networks are deploying data and using higher frequencies it turns out that a more ideal network would place the towers closer together than the traditional locations. This is causing massive reconfigurations of the networks as the carriers try to improve data reception.
Cell sites get busy. Or said another way, any one carrier on a tower might get busy while another carrier might not be busy. As cell sites get busy they do a couple of things to handle the excess traffic. Most carriers still give preference to voice over data, so as more voice calls are added to a network the amount of bandwidth allocated to data is often choked down (but not always). And eventually the tower site refuses to add new customers. But when sites get busy the performance normally degrades.
You might be roaming. Roaming is when a customer is riding a different network than the one to which they subscribe. If you are an AT&T customer and are roaming on a T-Mobile site, you will not get the same priority as a T-Mobile customer. This might mean getting slower data speeds if the site becomes busy, and it could also mean being booted from the site as it becomes full.
Spectrum is not created equal. There is not just one spectrum being used for cellular data. There are already nearly a dozen different slices of spectrum being used and the FCC is going to be auctioning more over the next two years. Every time you move to a different cell site you might be changing the frequency you are using. Carriers not only cobble together a network of the ideal cell sites, but they also play a chess game of deciding which spectrum to deploy at each tower. None of the carriers owns all of the different spectrum available, and the spectrums they own in different cities can be drastically different. This means getting four bars at your home might not give you the same experience as getting four bars when you are traveling.
What your phone allows. Perhaps one of the biggest differences in reception is that each cellphone maker decides what spectrum a given handset is going to receive. It costs a lot of energy, meaning battery time, for a phone to always search on all of the different frequencies. So different handsets allow different frequency bands. This is why LTE coverage differs so widely because there are many sets that don’t even see some of the LTE frequencies. All handsets look for the basic cellular bands, but only the most expensive sets are designed to look for almost everything out there. And as more cellular bands are allowed into the mix this will get more pronounced. Of course, you have to read very deep into the specifications of your phone to understand what it does and does not receive. Good luck asking that question at the cellphone store.
Plain old interference. Every cellular frequency has a different propagation characteristic. If you and the guy next to you are talking on different frequencies then you each will be dealing with a different set of interference. This is one of the reasons that cellular coverage is so wonky in complicated buildings like airports and hospitals. Each cellular frequency is likely to find a different set of problems in a complex environment and one frequency might get killed in a given part of the airport while another is fine. This is why you might find yourself walking around trying to find a signal while people around you are still talking.