It was a bit of a puzzle to read your annotation (a knight in English annotation is a N the K is reserved for the king)
The problem with Nxd5 is that you weaken the a1-h8 diagonal a lot. After the knight leaves c3 there is a lot of pressure on the d4 pawn. Since the pawn cannot really move (because of bishop Bxb2) white will have to make a stand on d4. Defending d4 will cost white a lot of time and his pieces will have to go to some weird places. Thus stronger is cxd5 where white keeps the diagonal closed and if needed can move his d pawn.
Example moves: 6. Nxd5 Bg7 (the treath is of course c6 and taking on d4) 7. e3 c5 (making use of the weak diagonal thus black cannot take on c5) 8 Nf3 cxd4 9 Nxd4 Nc6 (developing and once again hitting the white center please not that the knight from d4 cannot really move because of the strong bishop) and the white center is under a lot of pressure and the pawn on d4 cannot really be reliably defended.
In the game Varuzhan Akobian with white and our national hero Anish Giri with black, they played the Grunfeld-opening in the Al-Ain World Cities tournament in 2012 in.
After the moves 1.d4-Bf6 2.c4-g6 3. Kc3-d5 4.Bg5-Bg7 5.Bf6-Bf6 white played 6.cd5 But what about 6.Kd5 instead of 6.cd5 ??
There must be some refutation against the white move 6.Kd5, otherwise Anish Giri wouldn't have played this. But how?? Please, tell me.