Tom is right here. The Coach would like to assign you a heavier weight, but as there isn’t one available to it, the only way it can progress you based on the previous feedback you gave the Coach, is to increase the total training load by increasing the reps.
It’s essentially the exact same behaviour described here 
In your case you are maxed out at your 20kg pair.
Higher reps/Lower weights also builds muscle effectively as well, what is important is that the high reps are taking you to failure 
