Those are messages with very strong signal. It’s likely that you are receiving some messages with a signal even stronger that can’t be received anymore.
So you aim for about 5% of those messages and hope that there aren’t too many “even stronger” messages which can’t be received.
Or if you only care for planes far away you just ignore the percentage.
That could mean you don’t receive planes flying close by very well but depending on the case it can increase reception at least a bit for planes far away.
This is not guaranteed though, depending on electrical noise it might not help going higher with the gain.
Also see this thread:
Thoughts on optimizing gain