The same happens if you go second coil savage as a character with lvl60 or 70 weapons. The dreadwyrm weapons i135 are better than a synced down i405 weapon. It actually makes a little sense there because you shouldnt be allowed to get better gear than i130 because it was the max ilvl at lvl 50. But in ultimate i345 and i375 are allowed.
As you said the current sync system doesn't take 5 ilvl difference into account for weapons. Any weapon higher than i370 (when not diamond i375 weapons) will get synced down to i370.
I am unsure why you think it is only physical jobs that are affected, mage/healer weapons are -1 weapon damage too at i370.