What I've observed is that ReplayGain album gain is the same value for all tracks in an album (naturally as its the average).
I'm also using Foobar to calculate the equivalent itunes Soundcheck value for album gain. However, when I look at the value (COMMENT ITUNNORM), even within the same album it can vary. Should it, be uniform, as the ReplayGain value for album gain is uniform?
If I use dbpoweramp to calculate the soundcheck album gain, it as I expect (maybe I am wrong) calculates a uniform value for all the tracks in one album.
Edit2: Apologies, this might be better off in the foobar subforum. Feel free to move.