That's because different scalers are being used. The live scaling on your GPU is not as impressive looking as the scaler people use when they downsample their screenshots in Photoshop/GIMP with lanczos or similar.
A kind person gave me this game, and I hope to give back a lot of great shots! The in-game photomode is a bit limited, but better that than nothing. It has one flaw: it can't be used during normal gameplay, you can only enable it when you replay a mission. How stupid is that? I thought so too.