In Mathematics, what is the standing of the journal Proc. AMS?
Scimago is a clearly false, possibly semi-fake ranking. It is very easy to check this: for instance when I clicked on your link I got the following amusing result:

Vital and health statistics. Series 2, Data evaluation and methods research   is considered of higher ranking for mathematicians (!) than the Annals of Mathematics.

There is no need to elaborate more.