"They use the best carrier at any location as they are reseller of them all so they have that flexibility"
technically this is very doubtful. Net10 can have as many partners as they wish, but it has to set a Home PLMN ID(either Verizon or AT&T) in the SIM and the cellphone normally sticks to its home network as long as there's coverage.
unless NET10 makes his own cellphone which doesn't comply with the standard.