heres the difference,
the P90X, Crossfit, 300 crowd believes in order to get the most out of your muscles you need to "confuse" them by doing a different workout every session.
they claim this is the best method because they claim muscle memory picks up and adjusts via the natural process through homeostasis.
the biggest flaw in their theories is muscle memory, well at least how they use it. muscle memory actually has to do with set points (a terms thats been used a lot around here lately) and to give an example of what im talking about lets say you are 200 pounds and have been that way for 6 months. your body is pretty used to carrying the weight of 200 pounds therefore if you got sick and lost weight after getting better and regaining appetite your body would say, ok we took care of that now its time to get back to normal because your body thinks you NEED to be 200 pounds so you would eat and eat and if you were at 170 you could likely get back up to 200 in a months time, thats muscle memory.
when i say you get stale from doing a lift think of it this way, you cant infinitely progress on one lift. everyone says, change the variables (rest, TUT, weight, reps, ROM) but regardless you cant start benching 135 and keep adding 5 pounds a week, 2 weeks, or a month, forever eventually you need to change the stimulus. the problem is people jump the gun too soon, or sometimes they do it too late.
i would say once you start noticing a lack of progression on a lift, dump it, try something else til the same thing happens and then go back to it later if you want and see what you can do.