Without talking about component differences, since some people buy their bikes and ride them as is:
Were the bikes set up completely the same? Saddle position, reach, etc. etc.
Were the tires pumped to exactly the same pressure?
Were the roads exactly the same?
Were wind directions exactly the same?
Did you get tired (mentally and physically) as you went from bike to bike?
I could go on and on...
You never buy the last bike you ride because it wore you out the most.
We do, at our place try to control the weather as best we can...LOL...