There is a significant different between these two events. In numbers the man purposely went out to work, and did work - he broke the sabbath.
In Matthew the disciples we're just moseying along picking off heads of stalks of wheat and munching on them. They werent out their harvesting the field or something. It's more like you walking along munching on one of those little cans of Pringles or something
There is a pretty significsnt difference here. And this shows how insane the Pharisees rules had become. They considered the disciples actions "work" and it was not. They also had a rule for people that they should wash their hands up to the elbow before a meal, just washing your hands wasn't enough. The Pharisees were out of control, and there was a hug difference between the man chopping wood and the disciples picking grain heads.
I do see how it can LOOK like a contradiction though, it's a good find.