I flew premium economy this past summer, YVR --> FRA --> NBO; return flight was business.

Both my wife and I have E50, used eUPs to upgrade AC metal flight from YVr to FRA from premium economy (lowest) to J. It cleared for both of us, we copayed $500pp. We had a 3 day stopover in FRA (went to Mosel, very nice). When we arrived back to FRA for flight to NBO (Lufthansa metal) we were upgraded to J again, not charged and I didn't request it, at the check-in counter. Is this luck or fairly standard thing they try and do if you have already upgraded one segment? Does flying back on J have anything to do with it? I have had good luck with PY to J upgrades on these mixed cabin flights recently and I'm trying to get a better understanding of whats going on.