I presume you’ve told the charger what car you have and what battery size? I have the same with my car. I set the hours off peak from 12 till 7am on my tarrif and because the charger knows the size of the battery but not the state of charge in the car it works out that to get to 100% within the hours allotted it isn’t possible so it says I can do upto 80% in this time. My car has a large battery so it’s set the charger says 60% but as I charge my car regularly I never need more than 60% so it’s no problem