So from some of the stuff Marvel released we know it was Fury that made the job offer, not Clint. So maybe Natasha never had eyes on the S.H.I.E.L.D agent that tailed her and doesn't know who he is when she meets him? It could be she underestimates him as just another "redshirt" or they meet for the first time when they're both undercover (William Brandt and Natalie Rushman?).