Well... let me give a little more info and you guys can see what i'm noodling over.
Let me compare two extremes -- women I have direct experience with.
One was limited... orgasmed in oral and cowgirl only. And even then, in a very specific sequence. Also, she was a one-shotter... one orgasm then no interest in even pursuing another. So if you slipped-up and she came with oral, we didn't even try the cowgirl position that night.
Now the other one was the other extreme... orgasmed with oral, missionary, doggie, cowgirl, reverse cowgirl, from the side, legs over my shoulders, etc. etc. Very multiorgasmic too.
As for me, I actually go in phases. I'll go for 2-3 months where I can't get enough doggie. Then, for some reason, doggie won't be as appealing and I'll want more missionary. Then... sure enough, 2-3 months later, oral seems to be my big turn-on.
With both women, when it comes to satisfying them, I've found myself settling into "routines" for our day-to-day sex. For example, with the multiorgasmic girl, it was oral x 2 orgasms, missionary for 1 or 2 orgasms, doggie for 2 orgasms, then I'd come.
With the other woman, it was oral, brief missionary, cowgirl, missionary (for me, because she was rough on my dick -- not a good rider), cowgirl, missionary, cowgirl -- and she'd orgasm on the 2, 3 or 4th cowgirl.
So here is my theory (and it's just a THEORY):
Women can settle into "proven" methods of orgasm because they are predictable and reliable and familiar. It isn't that they are bored or boring, it's just that they enjoy the reliability that comes with using a proven technique.
Of course, the countertheory is that I'm just really bad to get into sexual ruts with my partner and lack the creativity to break these ruts on a regular basis!