A StarCraft: Remastered map supports no runtime scripting language at all, yet epScript plainly runs during a game. The answer is simple: epScript is compiled into triggers and inserted into the map. What the game runs is always, only, triggers.
Once you know this, several things explain themselves at once: why a function call is not free, why a loop costs something, why a variable takes up memory. They are all triggers in the end.
Ordinary triggers have no arithmetic. All you get is the number modifier on an action - SetTo, Add, Subtract - plus the bitmask parameters Blizzard's Elias Bachaalany added in Remastered. Every operation epScript offers, up to square roots, is simulated out of those pieces.
So the arithmetic is not ordinary arithmetic. Add wraps to 0 past 0xFFFFFFFF. Subtract floors at 0 however much you take away - 10 minus 200000 is 0, not a negative number. If you need negatives you have to build them; they are not simply there.
var a = 0xFFFFFFFF;
DoActions(a.AddNumber(5));
println("a == {}", a); // a == 4
var b = 10;
DoActions(b.SubtractNumber(200000));
println("b == {}", b); // b == 0