For instance, Soderling's success on clay does not seem to make much sense. He is has a pretty sizable serve, flat groundstrokes, and so-so movement.
Soderling's strokes are pretty spinny actually, just incredibly hard and spinny.
His relatively poor movement means he is effective on a slow surface when he has time to get in position to set up for a huge groundie. Same reason del Potro and Isner can perform well on clay.
Another example is Nadal on grass. His serve is arguably his weakest aspect. He plays a hyper-defensive game. He does not come to net very often (31 times compared to Fed's 75 in the Wimbledon final of 2008). He is capable of hitting flat shots, but is obviously much more comfortable with using topsin.
Are there any examples more puzzling than these two?
1) Nadal's serve is pretty good, it's just not always 215 kmh. He puts a lot of spin on his serve....the slice serve is most effective on grass. He doesn't get broken much on grass partially because of this.
2) He is basically an aggressive counterpuncher, certainly not hyper-defensive, unless he's playing a ballbasher.
His exceptional movement is one of the main reasons why he's so good on grass, where small steps and quick feet help him get in position better than most/nearly all players.
Topspin is actually pretty useful on a (medium-pace, fairly high bouncing) grasscourt like at Wimbledon, to open up angles from the baseline.
His slice is pretty mediocre, but he doesn't use it much, so it's not too much of a handicap.