99 is the max amout of units possible to be made
You mean fighting units? Or do you mean food units like pigs & cows? Or do you mean units of food? Which ever you mean, I don't understand.
45 is the current unit being made
Eh?
Ok let me take a stab at this. (I suppose I could puzzle it out in the game till something clicks but that sounds like a headache)
Humans can only have 99 units of things that move (soldiers, workers, pigs/cows) 45 would be how many you currently have? Why does magic not have such a limit? Or do they, its just invisible?
My other guess would have been that 99 would be how many stores of food you can hold and 45 would be how much you have stored now. But if that were the case, the amount you have stored now would increase by the value after + till it reached the max of 99. So that can't be the case.