If a 'good' matrix is found, it simply means that 40 of the first 60 glyphs from the subtitle have been positively identified. It can't get better than that. It means that the matrix in question indeed contains lots of glyphs for the current font, so that's the file where the glyphs belong.
The purpose of this is to put all glyphs for a subtitle in the right matrix. The perfect matrix file shall ideally only contain one single font, but no more. Nothing 'suboptimal' there.
Now if there were multiple 'good' matrix files, it would just prove that you have 'double' matrix files, i.e., more than one matrix containing glyphs for the very same font. But that's all there is to it.
If you don't check the 'auto-apply' checkbox, you'll see a dialog where you can decline to use that matrix. In that case, SubRip will continue the search and maybe find another one. But the match set will always be 40 glyphs, never more, because the very moment when 40 glyphs were identified, the FindMatrix function will stop immediately (for maximum search speed).