All the other Sharpness weapons are a straight +2 to hit, +2 damage.
Greataxe of Sharpness is +2 to hit, +2 to damage, and then an extra +1d10+2 lightning damage. It looks like it might be applying its modifier on top of the Stormblade Greataxe instead of the base greataxe item type.
Also, the extra +2 on the elemental damage is an example of what looks to be a double dipping bug across any weapon that has both a flat damage mod and extra elemental damage dice. For example, if you put Oil of Sharpness on a Lightbringer Greatsword, it goes from 2d6 + 1d8 to 2d6+3 + 1d8+3. EDIT: This appears to be a display-only bug. My Greataxe of Sharpness sometimes rolls 1's and 2's for lightning damage but never 11s or 12s.