Verizon Roaming on AT&T

As I was looking over the data for this month, I noticed that in one of the dead-zones that I previously reported for Verizon, there was a point-in-time when Verizon attempted to roam on AT&Ts network.  I only seen this with 2 data points (out of hundreds of thousands).  This did not occur last month.  I travel this area fairly often.  Whether or not I actually could use the service, I can't tell from this data.

I believe there are some parts of the country where roaming agreements exist between these 2 carriers, but my understanding was that it is only in specific areas where one of the carriers had coverage but the other didn't.  I didn't think it would happen in an area that both carriers already covered.

In the screenshot below, notice the Operatorname as Verizon Wireless but the operator is showing the mcc-mnc associated with AT&T (310-410).

What do you think?