Its easier for Mo (and Sadio/Diaz) to not be offside because they generally have the benefit of looking right across the line, whereas any #9 has to take a guess a little on what might be behind him. Mo had an offside where he definitely should have stayed on yesterday. That said, Darwin could just use what he can see as a guide as he probably has the pace to make up a yard on whatever is behind him.
Anyway, what I was going to say is I don't remember him being quite as offside from the left earlier in the season but I might just have forgotten