Negative side of waveform attenuated

Hi,

I'm using Audacity on a Mac to digitize my vinyl LP collection.  I noticed that on certain recordings, the negative side of the waveform appears to be attenuated.  In the quiet parts of the music, the negative and positive sides are symmetrical, but in the louder parts, the peaks go farther on the positive side than the negative side.  In fact, they never seem to go lower than -0.5.  They don't seem clipped, just sort of dampened.

Does anyone know what might be causing this?  Is it just a inherent property of some of the old LP recordings, or does it indicate a problem with my digitizing setup?

Any help would be much appreciated.