I’m pretty sure I’ve gotten counterfeit polo and Levi clothes from Amazon, but they are good enough that it’s hard to tell other than they seemed to wear out faster.
You may have gotten legit goods from the lower end lines of those brands. Both are terrible about have multiple quality tiers with minimal differentiation in marketing. The difference will be price, quality, and QC even if they look similar.
The Nike or Polo clothing sold there isn't overstock. It's legitimately lower quality items sold at a cheaper price. They're intended to attract price conscious consumers that like the brand.