To add more intrigue, the Lumia 925’s specifications on the Nokia Developer site confirm the US variant of the 925, RM-893, does support LTE band 17 (700MHz) which is used by AT&T. What do you think is going on?
We could grab at any number of reasons, but the simplest explanations are usually in the ballpark. If a customer bought one from T-Mobile outright and was not bound by an equipment purchase plan, it would allow them to use the device on AT&T’s LTE network. It is also possible that AT&T will offer the Lumia 925 to replace the 920 after the T-Mobile exclusivity period ends.
Specs from the Nokia Developer page show the Lumia 925 supports LTE on both T-Mobile and AT&T
Short of AT&T offering the Lumia 925 directly, it is not outside the realm of possibilities that Nokia would want to sell unlocked models after the T-Mobile exclusivity deal is over as well. Either way, it looks like folks on AT&T that think the Lumia 925 is calling their name might be able to answer.