Two things come to mind. One is air screw adjustment and ambient temps. Was the temp the same both days?
The other is age of your gas. I've found that if I don't use a full batch of premix in 2 weeks, I'll notice a loss of bottom end snap by the third week. I'm sure it affects top end HP as well, but the low end throttle response is where it's most noticeable to me. During the hottest 100 days of summer, I need to use it up each week or the response is very flat. :|
I think it has to do with the light ends boiling off in storage, even in a relatively cool garage.