mulled window question

[AMYGVHB]

Everytime I mull windows , there is a thin piece of trim material between the units that shouldn't be there.  This always happens, but this is the first time it's become painfully obvious because the window frame is black and the trim is cedar. The top of the bottom window and the bottom of the top window are at set to the same height prior to mulling.  See pic for reference.

Bottom window is a casement, top window is a picture.