I've found two reasons Verizon's map is different:
1) Because of the breakup of AT&T (landline) and then recombination / merger of AT&T (landline AND wireless) many towers across the country went from AT&T ownership to third party lease. All three companies that merged gave up tower leases to meet regulatory approval. Verizon's map does not include any third party lease towers.
AT&T was formed by partial mergers of Bellsouth, SBC, Cingular (a Bellsouth subsidary), and portions of the AT&T wireless network and the AT&T landline and Networking divisions.
2) In certain areas, such as my own, AT&T wireless operated under third party companies or MVNO's (Mobile Virtual Network Operator) such as Suncom Wireless. Suncom was purchased by T-Mobile in 2008. In MOST places in the country TMobile is on a separate band for the 3G wireless coverage - but in the south - they operate on the same band. AT&T & T-Mobile actually share a sort of data roaming agreement. Certain towers in my area (and I'm sure others) are "special towers" used to dole out signal (voice and data) to T-Mobile (former Suncom Wireless customers) and to AT&T (former Cingular customers).
Neither of these facts are taken into account with Verizon's "there's a map for that" maps.
Combined with the fact that Verizon used older 2008 maps where they DID have more coverage - this is the reason there's such a huge discrepancy.
Verizon is trying to "trick" all those iPhone customers that have complained about AT&T's poor signal. While I have had my share of problems with AT&T and had my share of signal issues - it HAS gotten better over the last year. Better than Verizon would have you believe.