First of all, hitting a few line drives in BP is nowhere near enough to grade someone a 60. It could cause a scout to note he looks like a 60, but most scouts will not actually grade until they have seen the hitter hit against live pitching. The grade would be based on many things besides results. A hitter I graded very high a few years back never hit a ball out of the infield while I saw him. Raw power is one area where it does help to see how far the ball travels. But it’s not always necessary to actually see it, to know it is, or will be, there.
To answer your other question… yes, grades can change based on what the scout saw that day. Grades can also change from one scout to another, both watching the same thing.
Grading is done based on the scouts opinion of a player/hitters ability. All things that pertain to ability and potential are taken into account. Usually there will be 2 grades –one for present ability and one for future potential.
To me… hitting ability is the toughest thing to grade accurately. There are too many variables and usually not enough appearances in the right situations. Every once in awhile, there does seem to be that, somewhat sure shot, above average hitter. Power, speed and arm are the easiest to grade. I think most scouts would agree.