Yes, that is what I mean.
Let me explain it slightly differently using a rice and cup analogy: each var is a cup. That cup is pretty big as it can hold ~65000 pieces of rice (0xFFFF, 2 bytes). Now, just because it can hold that many pieces, doesn't mean it has to. The cup could be empty, it could have just 2 pieces of rice, or it could have 30,000. It all depends on how many you put in it. Vars are like that cup of rice, each one can hold a number between 0x0 and 0xFFFF (~65,000 or 256*256). So when you use set var, you are filling the cup with that number after first dumping it out. If you use addvar and subvar, you will be adding that much rice to the cup or taking away that much rice, respectively.
More clear?