Didn't read whole thread, so forgive me if I'm rehashing what someone else may have said.....

A 2 pound difference between bikes does not equate to a 2.5MPH difference. In fact there are few things (or combinations of things) about any two road bikes with only a 2 lb. difference in weight that could make such a difference. (A 12 lb. difference and knobby/soft tires, MAYBE!)

I'd say the difference is you.

I know that there have been plenty of times that I've clocked a 2.5 MPH difference on the very SAME bike (I only have one now), on the same route (no traffic lights...rural roads), and under the same weather/wind conditions. Sometimes it's a physical thing; sometimes it's a psychological thing. -For instance, I suck at climbing- but for some reason, I actually found it easier when I had a standard crank, as opposed to the compact crank I have now. There's no physical explanation for that, other than that I must be bat-feces crazy!

Our bodies are not machines, and couple that to the fact that they are controlled by something which is not a computer, means that we can be unpredictable, and things aren't always what they seem (que: Twilight Zone music)