Why doesn't -3.0dB plus 3.0dB = 0.0dB?

I have noticed this several times when trying to finalize mix output levels - if I play my track all the way through and note the maximum peak level (say -3.0dB) I SHOULD get a peak level of 0.0dB the next time it’s played back by simply raising the fader by 3.0dB. It’s simple math!

But that’s NOT what I get. The peak output level shoots well above 0.0 by several dB. In other words, -3 plus 3 does not equal 0 - either the fader calibration is wrong or the meters are wrong.

Anyone else notice this?

BTW - I do not have any plugins on my 2-buss. There should be nothing but the fader and meter involved here.