Can someone explain this Tandem Breaker Setup to me?
GE does things differently
Since it appears you have a GE panel with GE breakers in it, your situation is a bit different than most folks.  What you see there aren't &quot;tandem&quot; breakers at all, but half-width &quot;double-stuff&quot; breakers (THQP) instead, with the top breaker independent and the bottom two breakers handle-tied to feed a multi-wire branch circuit.  A filler plate was then used to keep stray fingers out of the remaining half-space.
This means that the MWBC has a neutral wire landing on one neutral screw in the panel, and the outdoor lights circuit then has its own neutral wire landing on a different neutral screw in the panel.  As to why it was done this way? It was probably done this way simply because the installer had THQPs handy instead of THQLs; there's not really much to be concerned with about that, although occasionally a THQP will wind up losing its grip on the bus a bit and becoming wobbly as a result, which can be fixed simply by replacing the breaker.