I just don't get the 2 month after-the-flight calibration. I know that
their must be some logic to this but it escapes me.

Say I fly a great flight for diamond distance. My FR is out of wack by
1,000 feet (unlikely but possible). I send it in for calibration and
now it is perfect. How does that help the previous great flight?
Isn't the previous log file still 1,000 feet off?

I have to assume that I get a chart that says "Your FR was out of wack
by 1,000". I then supply that with the badge or record documentation?