As you create more you get better at it. By default, after you've crafted 10 of item you get 5% productivity on that recipe. Then, every time you double the amount you've crafted, you get another 5% productivity. For example, having crafted 250 of an item would give you 25% extra productivity on that recipe (10, 20, 40, 80, 160) and your next bump would happen at 320 crafted.

This can all be configured in mod settings, and you can view a GUI to see how much productivity you've got by pressing CTRL + P, or clicking the productivity shortcut.