If the Song Settings are fine, this could be an MP3Gain issue...

MP3Gain has several options for applying gain and could cause the volume differences depending on its analysis of the track. I only use MP3Gain to boost my tracks by a constant rather than letting the analysis take over. I found that some peak RD shots can alter the analysis and lead to unwanted volume differences between songs.

Check you BIAB Render settings - turning off Normalize and any Gain. Then listen to the WAVs and see if the differences are still there. You could then use MP3Gain to perform an Analysis only to see what the volumes are.

R