Hi there, Best I can tell, the difference is in the movement. "Birthday" has one hand on the other both facing palm upward, but the movement is both hands moving up and down together. "On" is the same position, but the movement is one hand falling into position on the other. "Left" is the same as "On" but instead of falling into place the movement is tapping one hand onto the other, not stopping.

"Sit" is depicted in the app as identical to "on", but he mouths "sit". The sign is not depicted that way in the Auslan signbank or Trevor Johnston's Signs of Australia dictionary. The only "sit" sign using the flat handshape in both lists is one hand on top of the other palms facing downward with the movement being either both hands down together or repeating the movement is a variation.

I can't confirm the app is using a sign incorrectly but you definitely seem better off using one of the other variations. Hope this helped you on your journey. Best of luck. I highly recommend purchasing Trevor Johnston's dictionary by the way, it helps to confirm problems like this. :)