Grav surveys tape place at grav points (specific spots in space around the star). It generally takes a little while to survey a grav point so the ship hangs around that spot until it is done and then it moves onto the next spot.
As for the one on Earth, it may be that there just happens to be a grav point on the location Earth occupies.
Now if the ships are hanging around those points and the SP number is not updating, something is wrong. It may be that they have finished the survey. Have you fund jump points? If so, they are awaiting orders. If not, I don't know what's going on (out of fuel, maybe?)