Not sure how much stock i would put into that. Doing what you suggested i chose the unlocked HTC One M8 and chose USA and it gave me the big2 plus Solavei and Straight Talk.
I know that Solavei runs off TMOs network and they ONLY show 1900 for Solavei. They are missing the 1700 / 1900 MHZ for LTE.
Straight Talk runs on ATT and TMO. Both ATT and ST they only show 850 mhz for 3g/4g and 700mhz for LTE.
They are completely missing the 1900mhz specturm ATT uses.
I can also confirm TMO uses LTE on its 1900mhz spectrum in Dallas. So they are technically missing 1900mhz off the LTE category listing as well.
But im nitpicking.