This is a really interesting question actually - and I don't think that simple differences in potency really explain it.

I have noticed as well that some weed seems to have a longer lasting effect and some a shorter lasting effect that doesn't seem to be relative to potency.

I can smoke a bowl of some weed and get ridiculously baked and then have the high pretty much dissipate in an hour. Then I've had different weed that I can get slightly buzzed off of a hit or two and notice that that buzz is still around a couple hours later - or get ridiculously baked and still be pretty high a couple hours later and still simi-high an hour after that.... There does seem to be a difference in the rate of effect decay. I have no idea whether this is a product of genes, cure, or what - but I do know that it can't simply be a higher initial THC load.