Gain fluctuation question.

Question:

Why do gains seem to vary so much on mixers?

Scenario:

I have two tracks, each are WAV 44.1 24bit, and each are all relativity highly compressed kid drum led tracks that are hard limited to 0db, so when the kicks are playing it’s always hitting 0db. In any DAW that I load them into they read 0db, yet when on a pioneer mixer (and other mixers too) the gain seems to vary. This is the exact situation I had at the weekend…

1) I load the first track, find the ‘loudest’ point, and set the gain on the mixer so it is as high as it can go into the orange LED’s without knocking the red LED’s once. It’s also worth noting that I have not touched the EQ, its set to flat. 

2) I then load the second track on the same channel, skip to the loudest point, and now suddenly it’s marginally louder and the LED’s are regularly flashing up into the red.
This also happens in reverse, so sometimes you can set the first track to not peak into the red and then the next track you play will then be quieter and be able to be turned up without entering the red.

Can someone explain to me why this happens? Sure we’re only talking small differences, but surely if the tracks are WAV and hard limited to 0db then they should always read the same on the LED meters and never need adjusting or pop into the red when they shouldn’t.

I’ve been a DJ for a very long time and have always tweaking my gains as necessary and will no doubt continue to do so; I’m just trying to figure out why this happens as in today’s digital age with such precision I would think I would be able to set the gains and forget them when we are talking about music that has been prepared and limited in the ways I’ve described above. Is there something I can do that would allow this consistency? As it would be nice not to have to tweak them all the time when the source files are limited to the same volume.