Most (not all) 50s Gibson and Epiphone P-90s are A3 or A5, or sometimes even a mixture of both. (I forget where, but I once saw a teardown of an old P-90 which had two A3 magnets stacked on top of each other on one side and an A5 on the other; quality control was not really A Thing in the 50s.) I assume SD went for A2 in the Phat Cat because A2 is much cheaper/easier to get hold of than A3, or at least it was when they designed that pickup, and the difference between A2 and A3 really isn't that dramatic, especially when you have two of them as in a P-90. But I do think most people think of the A5 as being the P-90 tone, anyway.