I'm not sure tyre sizing is an exact science - not in the width dimension anyway. Pick nominally identically sized tyres from different manufacturers and they'll have different rolling radii. So your 120/90 might well be the same as someone else's 120/80. Or it might be loads bigger ...
If it's that critical you could buff off a few mm of tread or reduce the number of revolutions / mile by running it half flat. Both of those are essentially short term solutions though
Personally if it was the tyre I wanted and it didn't rub or catch when fitted I'd go with it. Give it a few thousand miles and the problem will solve itself.
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