Assuming you did the test right (and you probably did), then it could very well be you have a "melty" piston. Overboost, ignition timing too advanced (or not retarded enough on boost) can lead to detonation, excessive heat and the pistons will deform. The piston rings also will shatter, but leak down can still be very good even with the rings in 25 pieces.
This is from my engine...44% leak down on this one. You can see lots of oil build up on the piston crown. Look closely and you also see that the top ring land is deformed and much larger than its supposed to be. The 2nd ring land was squished so the the ring would not rotate.
This is from a friends engine...same deal, but a bit more obvious.
Both of these cars (930s) ran very well (not smoking out the tailpipe.)
The below pic is from another friend's SC. Piston chunked by severe detonation. This car didn't run very well and was puking oil from the tailpipe
