One of the things about the Firstbeat Metrics is that many of them are based on Excess Post-Exercise Oxygen Consumption (EPOC). This is a method that tends to under-emphasize time spent purely in the aerobic zone, and give greater weight to time spent in the sub-threshold and above zones (at least compared with TSS). Unfortunately Garmin doesn’t give any visibility to the raw EPOC numbers, but I did get a good understanding of how they worked with an older Suunto watch.
This may well account for the the different RT metrics from your ride.
VO2Max is also a factor in the EPOC and Recovery Time - a high VO2Max (as estimated by Garmin) will result in a lower EPOC and Recovery Time estimate for the same workout., so if your VO2Max changed between those rides, this may also explain things.