Difference between burn in timecode and embedded timecode

Hello everyone,
While trying to figure out the source of latency in a virtual production studio, I’ve activated both burn in timecode option in the AJA media source used for outputting a cinema camera and the embedded one in the SDI signal outputted (still in unreal).
Then I’ve figured around 15 frames of difference (at 50fps) (embedded timecode is late compare to burned one). The timecode is the default one generated by unreal. Do you know at which step this timecode are taken (exemple before the render and after) ?

Best,
Alexandre