There do seem to be quite a few exceptions to this. And the Latin root of
'-anus' only implies 'of' or 'pertaining to', not particularly distinct from
'originating from' (cf. 'montanus', 'montana').

True, and a lot of it is custom, rather than grammar.

Acer pseudoplatanus - in honour of a fake philosopher
Matthiola incana - in honour of a lost civilization
Galeopsis ladanum - in honour of a triumph of Soviet engineering

Absolutely. There are some real ones that are a lot worse. Lindley named an
orchid genus Aa, just to get it in the front of Index kewensis.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)