I know, this topic comes up all the time, because it seems like every pregnant woman thinks she's carrying twins. ;)
I have three daughters - all unassisted pregnancies and unassisted homebirths, and I didn't measure fundal height so I don't know how mine compares, but I *do* know I have fairly small babies (I'm under 100 lbs. when I'm not pregnant, and hubby's small, too) and I have a long torso, which apparently makes it easier for baby to "hide" and can make the fundus measure a bit small.
I'm 32 weeks, measuring just under (by about 2 cm), and we haven't been able to palpate a back or butt in months - just one head. But there seem to be limbs everywhere, and although I seem to be carrying lower and more out-front than in any of my other pregnancies, I've also been feeling like I have a lot more baby in my guts and up in my ribcage, which doesn't seem to make sense.
I'm not necessarily saying I think twins are likely - but it is theoretically possible to miss twins through palpation/heartbeat alone, right?