Actually no, they are exactly the same difference -- a difference of 5 quality points per 20%. That part is a linear relationship.
Now, the relationship between quality/stat points to actual performance could be non-linear. So a 60% team may do worse against an 80% team than an 80% team do against a 100% team. That part is unknown and probably involves too much noise from the random number generator.
But the percentage is introduced to spare some managers from getting headaches for translating the numeric quality value to their current level's five-star standard.
A 23% difference translates to about 5.8 quality points difference. The poster's team was labelled as the favourite and still lost.