Hi Jaba. Merry Christmas to you and yours.
It may well be the extra ROD is attributable to these things
I'm only a mug playing a guessing game, but I assume the RoD came from the Mode C radar data and is independent of the airspeed.
My understanding is that the altitude readout is given in increments of 100 feet on a controllers display and I don't know if the raw data that is recorded is more accurate.
I see little problem in believing that within those parameters, the RoD data is probably quite accurate.