What is up with RTA ranking and matchmaking?

I have been trying RTA for the first time this last month, but I am confused as to what to what monster/rune level is supposed to belong where when it comes to ranking.

I have just reached F3 am not able to get higher atm More than 50% of the opponents I meet have a full team that outspeeds my 291 speed starter when I have speed lead and they don't.

Occasionally I meet someone at my own rune and monster level where the match usually goes pretty evenly, but often-times I get absolutely crushed and outspeeded in F2 and F3.

Can someone explain to me roughly what rune/player/monster level belongs to what rank in RTA so that I can get somewhat of an idea?