Sublime Versions:
MacOS (Yosemity): Build 3065 (stable)
Win7: Build 3082 (dev)

show_at_center shows different behavior on the two operating systems.
But first of all, what I would expect from this command: Current line is centered in the editor window regardless of how many lines are before or after that line.

On Windows this only works if there are enough lines before the current line
On MacOS this only works if there are enough lines before and after the current line.