No elevation change? I'd guess it was either 0" up, or 0" down, and yes, there is definitely a difference....usually a foot's worth of speed between those two.
For instance, on tournament greens, if I have a putt of 6.8 ft that's 0" up, I'll hit it about 8ft, whereas that same putt 0" down, I'll hit about 7ft.
It also helps to extend your putting line past the flag and see if it changes from 0" up to 1" up (or to 0" down, etc.), and how far past the flag this occurs. For the same putt mentioned above, if the elevation changed to 1" up at the 7.5ft mark, I'd add a foot and hit it about 9ft.
Also, I think the elevation might be rounded down(?). In other words, 1.8" up will show as 1" up. ( I think? Icon can probably confirm that one...) It could also be that the change is at the .5 mark. (ie: 1.5" up = 2" up, and 1.49" up = 1" up)
But having said all that, Congressional's greens do seem a bit slower than one might expect for tournament speed. Don't worry though....if you make it through qualifying and get to the Open, the putt you described above will go about 15ft....lol