"frames per second".. what is: "estimated"

[BETLOG]

Sfter pushing shift-I one of the displayed stats is "frames per second" followed by "estimated"
I've noticed that video I recorded of a game at 60fps shows an 'estimated' of 30fps, and yet others have previously shown as 60 and 60.
What does estimated actually mean? Why would it show half of the intended framerate?