There was no way to anticipate exactly how hungry the Verizon customers would be to get their hands on the iPhone 4.
The Verizon iPhone was a rumor for the first four years of the device. While AT&T had a commercial monopoly over the device, users from other carriers desperately hoped that the king of all smartphones could be ported over to their plans so they could stop pretending that the Blackberry and Android were in any way comparable. When the iPhone 4 was finally released on Verizon, sales shot through the roof. It served to reason that AT&T would still be the main supplier of the iPhone 4 since it was available for a full eight months before the Verizon iPhone 4 and already had a slate of people that were already committed to the iPhone and were resigning contracts with it. Even with this the current numbers put 32% of iPhone 4 users with Verizon, a number that nobody could have imagined would be so high when it was given by the research company Localytics.
This number has the possibility to grow even higher, though AT&T will still lead for the foreseeable future. In the first quarter Verizon sold 2.2 million iPhone 4 models, while AT&T sold an impressive 3.6 million. It has to be kept in mind that AT&T has a longer foundation with the iPhone for its users and people are more willing to upgrade to it through their AT&T service since they likely have been dealing with it for longer, but this is the same situation that will be accessible for Verizon in the next couple years. The other thing is that Verizon only offers the iPhone 4 while AT&T offers the cheaper iPhone 3Gs, which will actually decrease the sales of the newest iPhone model in the long run. Verizon may begin offering the iPhone 4 for cheaper once the next generation model is out, but there is not telling of this right now.
The real tell will be how the iPhone 5, or the iPhone 4s if the rumors are true, is released. If Apple does indeed release an LTE iPhone on a full 4G network in September then the numbers will jump through the roof for them, possibly in a way that will not allow Verizon to recover in the next several years. If they continue to delay the progress of their network then it will likely assume that Verizon will have the first LTE iPhone next year, especially since their network is much closer to being completely 4G ready than AT&T's. The reality is that the numbers will remain fairly constant for the iPhone 4 this year and the real battle between the two companies will only begin once the iPhone upgrades its network capabilities.