Well, lending new money (i.e. an increase of lending over previous lending) does expand money supply and create inflation. If we look at the macroecomic aspects of money supply, most of current money supply is created through lending.

The key difference in your estimates of supply is that you include the future repayments of current lending, but ignore future lending. The general assumption in macroecomics is that we don't treat such lending as isolated one-off events, but as a sum of ongoing activity by many people, continuing forever at a stable level unless some event affects it.

Assuming that the principles governing lending don't change, the future repayments are balanced by payouts of new loans at that point of time, and the current payouts (if they are greater than current repayments of past debt, i.e. there's a net increase) are not balanced and thus increase the supply. If at some point the fundamentals change so that the lending decreases or stops then that event would decrease supply back to where it was.

I.e. if you often lend money to your friends so that usually someone or someone else owes you $10, then this lending is not a change and does not affect supply, but if you did not loan money and now you start lending, then that $10 is an increase in money supply.