@Mayne I'm not sure how old you are and what your TT levels are. I'm assuming you are fairly young with normal TT levels. I'm in my 40's with low TT levels, if I don't cruise between cycles I lose 95% of my gains in 12 weeks. I can keep my body weight up by eating but it's not muscle, I'm adding fat. Does this apply to everyone, no of course not but it's definitely true in some cases.
Also I think you aren't considering that it's easier to hold on to gains the more you cycle. If you have been cycling for years your body is more accustom to the extra mass and it's easier. You don't remember your first few cycles where most of it didn't stick? After a few I'm willing to bet it was easier to hold on to LBM.
Lots of factors go into it, it's not a black and white issue