Apple is throttling the Qualcomm modem in Verizon and Sprint iPhone 7 for parity with slower Intel modem model
According to analyst Jan Dawson who's familiar with the matter, the decision has to do with Apple's supply chain more than anything. The company chose to use the two modems to differentiate its suppliers, which carriers had no real say in. Moreover, marketing the same phone as faster on Verizon or Sprint's network because of these matters would have been unfair to competing carriers and users themselves. But in testing, the Qualcomm modem model was found to be slightly faster than the Intel modem model, although not considerably so.
In addition, Apple might be considering the increased power consumption of the non-throttled Qualcomm modem in its decision.
source: recode.net via Apple Insider