A way of explaining it that (hopefully) makes it clearer:

Let's say you have just invented the concept of prison, and let's say 50% of people come back every year after going to prison for the first time, and 50% never come back again.

In year 2, you'd observe that of the year 1 population, 50% returned, and 50% didn't.

In year 2, you'd also add a new influx of prisoners.

Therefore, in year 3, you'd observe that of the year 2 population, 75% returned and 25% didn't.

But, the real underlying recidivism rate is still 50%, not 75%.