Can look at it either way though - LBW review could be shown to be clearly hitting the stumps but umpire has given it not out. On review, would still be not out, even though the stumps were being hit (by maybe 40% of the diameter of the ball) as it'd be 'umpires call' which is essentially the margin that makes the review system there to cater for 'clear and obvious mistakes'. The offside wasn't a clear and obvious mistake and would have fallen into a 'ref's call'. Football are trying to make everything black and white which simply doesn't work..............
Off side though, (within the existing off side laws) IS black and white. Regardless of how tight it is. It isn't about it being 'clear and obvious' that she WAS off side. Once the images have proved that she was (however tight) then it is CLEAR that a mistake has been made.
Back to our LBW dismissal - let's say the batsman got a tiny feather of an inside edge on the ball. That wouldn't be 'clear and obvious' to the umpire in real time, but once proven, it is still CLEARLY an error.