Jermain Defoe is a very good example of the type of player you mention. Always good for 15 goals a season but when you look closely they were in the form of braces or hat tricks against the likes of Hull, Wigan etc. or scoring a couple to finish a game off. Very rarely were his goals the winner against Arsenal or Man Utd for example.
He was actually who I was thinking of when mentioning it, but I didn't have the stats to confirm if it was true.
Another was Anthony Stokes in the SPL right before we signed him. Everyone quoted his goals-to-games ratio, but it included hattricks against weaker opposition.