The phrase "extra raisins" usually means getting more of something, like an additional bonus or advantage. It suggests getting more than what was expected or typical of a situation. Full definition
I made the granola a few days ago, but couldn't keep my hands out of it, so I was 2 cups short, so I added extra raisins and extra coconut. (chefambershea.com)
If desired, you can press a few extra raisins onto the top of each cookie. (celebratingsweets.com)
And, though I didn't sit with box in lap, picking out the raisins to snack on, everytime I ate breakfast, I would dig through the box to get extra raisins for my bowl. (oatmealwithafork.com)