Estimated print time vs real print time

I am currently using Cura 13.06.04 to slice and print my items. Ive used other slicers and interfaces too, just prefer Cura right now. The other slicers were off in printing times as well.

Anyway, I slice an object and it says that it will take XXX time. The printing takes considerably longer, usually between 1.5x to 2x the time.
Now I'm not terribly worried that it takes more time because the prints are turning out great. So I don't think its a printer calibration issue. Its tuned up pretty nicely.
I'm just wondering if its something anyone else has run into this issue. Is it an issue with my machine not moving at the speeds the slicer is thinking it is, or just bad calculations of time in the slicer?

I believe this is because of acceleration, basically in Slic3r there is a setting to adjust the amount of time it takes, by adding on a certain percentage, if you use slower acceleration it will take longer.

basically the slicer it estimates the time, based on the gcode, but the printer actually has its own acceleration setting, in the firmware, so the slicer can only estimate the time assuming there is instant acceleration. for instance if you have a very organic print, the speed can remain pretty constant because it doesnt need to slow down around the curves. however a more square structure would require more acceleration and deceleration because the corners will be sharp.

Yeah, that makes sense.
Come to think of it, the items that seem to end up the closest in time are things like round single wall vases. I never thought of an estimated acceleration on square/sharp edged pieces.