So, on the pump days, the reps are higher. So, you aren't going heavy. But, you want enough weight that's it's still a challenge to complete all the reps.
Here's a paste from Christian's post:
The goal is to be able to do all the work sets with the upper limit of the range (8 in our example) with the same weight. When you're able to do that, you increase the weight at your next session. If you can't get 8 reps for all of your work sets, that's fine, but it means that you'll keep the same weight during next week's workout.