Hi,
this is resampling not pan-sharpening method. It means you have 10m spatial resolution but pixel which had 20m resolution earlier (in your case is band 6) is divided on 4 pixels with reolutions 10m but with the same values of grey.
If in SNAP choose resampling method Nearest Neighbour you obtain square pixels but when you change to method bilinear or cubic pixels are smoothed
Regards
Wojtek