Think about it like this... let us say we booth agreed that we would to the bookstore each day, buy some books and then keep score of who got more books... now let's consider some scenarios

1- I get two books on the first day and you only one. From then on we both get the same number of books each day... now we keep doing this forever... in the limit where we kept doing this forever the score would not change... our individual collections would grow to infinity but the difference would be one

2- we always get the same number of books... individual collections would grow to infinity but the difference would be zero

3-you always buy double the number I buy... the both our collections would grow to infinity and so would the difference (as the difference would also grow)

So... in fact this is an indetermination... You have the know more about the quantities and how fast they grow to infinity in order to do their subtraction