This question seems to come up over 'n over; over 'n over I'm surprised that people are surprised that, for commuting purposes, slicked mtbs are nearly as fast/as fast/faster for a specific rider on a specific route under specific conditions than 700c road bikes or hybrids. There's just too many variables to say, as many do with great certainty, that a roadbike is 'faster' than a slicked (I would say 'roadified') mtb.
We're not talking about riding long distances under 'open' conditions here, possibly in a group. Under those conditions, yes -- a roadbike will travel further/faster for a given rider output; that is primarily down to the more aerodynamic riding position, which comes into play at higher sustained speeds. Except at (near) road racing speeds, where the factor is wheel aerodynamics (i.e. narrow), it has nothing to do with wheel diameter, except insofar as that affects effective gearing.
So, to the OP: doesn't surprise me at all that you found a 700c hybrid 'slower'; the hybrid could very well have a more upright (less aerodynamic) riding position than your mtb; and its wheel/tire combination could very well be heavier, and have higher rolling resistance, than your mtb wheelset. Of course, you might also simply have been feeling better on the day of your personal best, which just goes to show once again 'it's not about the bike'
