There are two variants of the TIE/In, an original and a Mk2. The first one had four wingtip mounted SFS L-s9.3 laser cannons but was later modified to use four wingtip mounted blasters and two ball mounted L-s9.3 (or perhaps L-s7.2) lasers cannons. The ball laser configuration was shared with the TIE/ln Fighter at the time, an upgraded version of the original grey TIE Starfighter. Better cooling allowed the use of more powerful laser cannons, and equipping the ball cockpits identically made servicing both types of fighters easier on one Imperial deck crew.
At least, that's our retcon anyway. Doesn't really seem to conflict with any canon information (or cannon for that matter

).