Go into Options - Multiplayer settings and you can see that the scaling for chat opacity and scale don't meet up at 50 percent
MC-12477
Chat options not equal at 50 percent
Resolved
Duplicate
0
Kelsey
(Unassigned)
Unconfirmed
Minecraft 1.5
Retrieved
Opacitiy range goes from 10% to 100% that's why 50% is not the middle.