Blender confused as to current frame

Blender graph editor thinks it’s frame 215
Blender timeline thinks it’s frame 215
Blender frame control thinks it’s frame 217

How on earth do these numbers get disconnected from each other? As it stands when I tell it to render frame 215 I get a file called 215 but it’s actually frame 213. Having tested a little, the frames match to start with, but somewhere between 30 and 50 an extra frame gets put in, and then another just before 100. The exact frame this happens in changes as I move back and forth along the time line.

How do I resync the frame numbers?


Post the blend file please, could be many things…

Cheers, Clock.