GarageBand advanced preferences auto normalize by default. You may want to clear the checkbox.
Auto normalization may be a good thing if your record levels are too low. Yet if any of your record levels are deemed to be too high, this feature lowers all levels for the whole song.
When recording I try to get a good signal, about as loud as it will go without distortion. Bass guitar can be hard for me to keep under control. But I would rather have maybe one bass note come out distorted than have the whole piece dropped 10dB.
Figments of a lack of imagination