5.1 software decoding channel levels

[DrLex]

When using a USB 5.1 sound card with VLC, I noticed that the relative channel levels are different between movies, making calibration of the surround set-up a pain. If I calibrate the set-up with an equal-volume-on-all-channels test sound, it does sound correct for a certain share of movies. For other movies however, it is necessary to more than double the volume on the rear channels to make it sound correct. And sometimes the center channel appears to be too silent.
Why is this the case? If I understand correctly, channels in an AC3 stream can be recorded at different levels, but parameters in the stream indicate the relative gain levels, and an AC3 receiver is supposed to scale the channels such that the analog output is at the same reference level. I'm new to surround sound but I can't imagine that everyone with a home theater needs to do trial & error volume knob fiddling at the start of each movie.
Is it possible that VLC's software decoding ignores those gain parameters and outputs the raw channels at their recorded levels?