I think it just depends on the store. I went into a store that I've never been in (although I knew it was there), I just happened to be going to my credit union which is in the same plaza. I asked if by chance they had any 6+ in stock, and they said only 128 GB. But, you'll also get plenty of stores that don't/never had any. Not sure how T-Mobile is distributing them.
So it wasn't in a mall? I wondered if maybe stock might be better in malls where there's more competition or something. My local plaza store never gets anything.