screen saver changes by itself

My selected screen saver (which is a standard one on the 10.6.8 system) for my main account, disappears after I switch accounts and switch back. That is, for example, from my own account, I log in briefly as Administrator, and then return to my own account, and I end up with my selected screen saver having been replaced with a flat blue screen. Not a big deal to reset it, but why is this happening? It's actually been doing this for a long time ...