As others have pointed out, Fangraphs has two different FV scales, one for prospects and one for MLB players. The 80 FV/7+ WAR mapping applies to MLB hitters and not prospects. They definitely shouldn't have used 'FV' for both scales, but it's not hard to tell they're different if you compare any level other than 80 FV on the two scales:
A 70 FV position player on the prospect chart corresponds to 12.5 WAR in the first 6 seasons on average, whereas a 70 FV on the hitter chart implies 5-7 WAR per season. Clearly they're not measuring the same thing.
I mean, I posted the explanations FG gives for those charts. You can read the articles I linked. They make it clear that FV is one scale about projecting future value. Like, I do not understand why people won't just look at the words FG is actually using.
The difference is that the first chart is
retrospective and the second chart is
prospective. That is,
the first chart is not a projection. It's not saying what people thought those prospects would do. It's what they did in fact do.
The second chart is actually telling you what a projection of FV grade is supposed to mean.
That said, I agree with you that their descriptions don't completely backmap to the surplus value data. This is a valid criticism of what they are doing. But it also kinda makes sense when talking about projections. The pool of 70 FV value players includes both (a) the guys who actually meet that projection, and (b) the guys who fall short. The average of the whole pool is obviously going to be lower than. How to handle that problem is an issue every system is gonna have to figure out how to deal with.
Nevertheless, these are not different scales. FG already has a scale for present value - it is called WAR. In scouting parlance, this could also be called PV or "Present Value." FV, which obviously is "Future" Value, is a projection about a player will do in the future. You could put this grade on a prospect. You could put this grade on a young player. Because FG is linking it to service time, it would weird to use on a midcareer guy. But the grades are measuring the same thing:
what do we expect them to do during their first six years. See here where they put an 80 on not-a-prospect Aaron Judge:
Ranking 2017's Graduated Prospects.
Now, this makes sense given their definition of 80 FV in their prospect methodology. Judge was coming off 8+ WAR in his first season. It is fairly reasonable to project him as a 7+ WAR player going forward. You will note they are also giving a PV rating. This should really make it clear that FV is about prospect projection, and there is not some 2nd untethered FV for MLB players.