The biggest difference is likely the gearing.
To check that, ride your C-dale in the same gear as your fixie and check out the speed difference. If there is any, that difference may be from a combination of:
2. Pedaling technique
3. Bike weight (depending on hills)
5. Riding position
Day to day, weather patterns (air density, headwinds, etc.) can have a huge impact on your speed. That's one of the reasons I don't even have a speedo on my commuting bike. Who cares?