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ATT is GSM, so something is wrong there.
Because they also resell VZW's service on their proprietary handsets (CDMA). VZW's coverage is good, but the handsets they sell don't utilize their 4G-LTE network. If you buy one of their SIMs, it is at&t, you can use any GSM handset, (even ones locked to at&t or t-mobile(which you have to specify when you buy if it is t-mo) and you can use the 4G-HSPA+ network which is nationwide.