The differences between the Nokia Lumia 520 and T-Mobile's exclusive Nokia Lumia 521 have to do with the size of the device, which is a bit more than 4mm taller on the latter device, and the UMTS connectivity on T-Mobile's model. The Nokia Lumia 521 also has some differences when it comes to hearing aid compatibility.
Making the Lumia 521 slightly different might have been necessary in order to escape AT&T's exclusivity in the U.S. And the same thing might apply for the Nokia Lumia 1525. By making the Lumia 1525 slightly different in comparison to the Nokia Lumia 1520, it allows T-Mobile to offer its own exclusive version of the phablet, without breaking AT&T's exclusive rights in the states for the Lumia 1520.
We don't believe that this version of the Nokia Lumia 1525, is in any way related to a model with the same name that was rumored to be coming this year, as a sequel to the Lumia 1520. The speculation was that this phone would feature a 6 inch screen with 1440 x 2560 resolution, a 490ppi pixel density, and have the Snapdragon 805 under the hood. Not only was the rear snapper said to be 25MP or 30MP, solar charging was also rumored for this model.
Now that T-Mobile is selling only refurbished versions of the Nokia Lumia 925, and has started offering the entry-level Nokia Lumia 635, the mobile operator is looking for a top-shelf Windows Phone model to present to its customers. Unfortunately, the tweet from evleaks did not mention any time period as to when we might see this model made available.
source: @evleaks via WPCentral