The value of a currency is not purely a function of supply. It changes based on the relative values of the things you can buy with it, including the relative values of alternate currencies. Human emotions play a large part, too. Simply coming up with a number for the inflation rate is a complicated and inexact exercise. Actively controlling inflation is even more difficult.

A little inflation is generally considered a good thing because it incentivizes people to spend their money rather than hoard it.