Transactions with low fees always failing in Metamask?

Even though I say "always", I only had a few experiences with them failing in the last 6 month, but it was each time I used a low fee, like 1.01 Gwei. Why does this happen? From what I see on, even transactions with 1 Gwei should confirm in a median time of 10 minutes.

  • wth is wrong with /r/ethereum, almost nothings gets approved there anymore; I tried posting there too