the best way i have ever heard this described is thusly.
bulking is a different "hormonal milieu" than cutting.
different hormones play to differing degrees in the two processes.
it is not "impossible' to gain muscle w/o fat or to lose fat w/o muscle.
but it is very difficult, and it is much LESS *inefficient* to try to gain muscle w/o gaining fat, and lose fat w/o losing muscle.
for those who are not hormonally um... "enhanced" this is especially true.
for example, when one bulks, a good rule of thumb is that you will put on 2-4 lbs of muscle for every lb of fat you put on.
when one cuts, you will lose 2-4 lbs of fat for every lb of muscle you lose.
if one tries to gain muscle w/o fat (very difficult - but easier if one has high natural T and IGF levels, like many 18 yr olds d0), one just makes MUCH slower progress ceteris paribus
8 weeks bulking followed by 4 weeks cutting will in most cases result in a greater LBM than 12 weeks trying to gain muscle w/o some fat gain as well.
that is because per unit time expended, more muscle will be gained while bulking if one accepts SOME fat gain.
it's all about efficiency, end results, and cost/benefit analysis.
individuals vary, of course, but these are general truisms.
