Mystery solved
Looks like the rover did it on purpose.
MAHLI took the close-up images of a rock target named “Sayunei” on Jan 22 (Sol 165), located near the front-left wheel after the rover had driven over and scuffed the area to break up rocks in an effort to try and expose fresh material, free of obscuring dust.
There does look like there is a scuff mark on Rob's pic.
