Ah, got you. That could be it, but then why aren't there artifacts like that in the USD-BTC chart? On that one the prominent trade amounts are 1, 0.1, 0.01, ... as you would expect.

Could be that for USD-USDT algorithmic trading is dominant, vs, human trading being dominant in USD-BTC, in which case arbitrary amounts like 1, 0.1,.. would be less common for the USDT case. I'm not sure how to assess this though.