Logic Pro 9 Output flutuation

I've been working on several songs for quite a while.

I've noticed that the Output volume on audio and Midi rhythm guitar tracks varies.

If I start playback from the beginning of a song I get one volume on a later part of a song.

If I begin playback right before the recorded part, I get another volume.

(Starting playback a measure or two earlier can result in different volumes.)

I don't have very much track automation.

The sliders in the mixer are not moving.

Is this how Logic Pro works?

It makes it very challenging when doing a mix to guess about where the oveall output volume of a song will be.