I'm a bit baffled at how additional taxes/fees are calculated when using miles to upgrade. Three very recent examples below, all upgrading from PE to UC for various flights between LHR and BOS/JFK :

Called in June to upgrade BOS-LHR-JFK in August - additional payment $370.00.

Upgraded same day LHR-JFK at LHR airport ticket counter in June - additional payment GBP54.00.

Called yesterday to upgrade JFK-LHR-JFK in January - additional payment $480.00.

Observations:

1) Payment calculation seems random...?
2) Surprised at high cost of yesterday's transaction ($480)
3) In comparison at-counter payment in LHR looks (GBP54) like a mistake in my favor...?

Can anyone shed any light on the basis for calculation and whether my experiences seem reasonable?