stretch vs creep....
one is recoverable one is not (I can never remember which is which Chad, just plume gave up trying to remember which was which LOL).
Like was said some materials do it more than others....my skinnies have really stretched a ton after building, some of that is probably my redneck method of trying to make a string more than anything. Once it settles in all strings are going to do this to a degree. If you're bow is picky you may notice it in your shooting, you may not, more likely if you're close on fine tuning....if you leave it strung it'll bottom out and stop moving all together.
I also rarely unstring. Something to consider, stringing and unstringing is where most problems occur. If you shoot a bunch leave it strung unless you're in a hot environment or direct sun that could heat the resigns up and delam your bow (or you're shooting a all wood bow than dont leave it strung!).
if your bh is changing....your string length is changing also.