Thanks for the informative reply!
"Post every round" is the original concept of the USGA, AFAIK. As I said, us Germans have always only used (supervised?) tournament scores, plus scores from special private rounds with a marker, for handicap purposes. This hasn't changed.
Me, too, fell victim to the "few rounds trap". Below five scores (plus the old handicap), only the best SD (stroke differential) is used as the "average", and below three scores (plus HCP), two strokes are subtracted. Thus, with any SD higher than your HCP, the latter will drop by 2 for the new WHI. (Unjustly, those who posted no rounds will be spared of that)
Your pal and I obviously share the additional "treat" of one SD lower than the HCP. In my case, a 13 turned into 11, he must have had a whopping 8.5 (39 Stableford?).
It will cost me a few beers, I'm afraid :(