Having the dock on the right (or left) side of the screen, and using the full screen shortcut leaves a small gap between the window and the Dock. This doesn't happen when using the shortcut to move to the half where the Dock is.
Using Right Half shortcut:

Using Full Screen shortcut:

Yep ! I confirm that!
I thought this was only me..
I investigated this a bit and it looks like the result of rounding in the window calculations. My guess is folks with different display resolutions may or may not see this discrepancy.
@alvarosevilla95 and @ttben I might have found a workaround for this issue that introduces one additional move to the left (a bit of jiggle) but did fix a similar issue I saw.
Let me know if you want to try a test build to see if it fixes your problem too.
I have similar issue but not related to dock. I use ITerm in full screen often with multiple tabs etc. If is resize Iterm by hand or with Spectacle, it doesn't cover the full screen. But for some reason only on Iterm. The only way to get it back to where it's not in weird 98% fill is to close all tabs / windows and launch a new one. In first image, it's subtle but annoying because I use a dark window, light background shows at bottom and sometimes sides. Playing with Iterm settings has no effect on Spectacle behavior.
After Spectacle full screen:

Fresh launch Iterm before ever using Spectacle full screen:

From some Google searching, the dock reserving 5 or so pixels has been around for a long time.
@worriorbg I would like to test your fix.
@lacostenycoder https://github.com/eczarny/spectacle/issues/280#issuecomment-236540753 may help you out
@jakeonrails wow thank you so much!!!
Most helpful comment
@lacostenycoder https://github.com/eczarny/spectacle/issues/280#issuecomment-236540753 may help you out