It is very much like comparing apples and oranges. For Primecoin, the integer part of difficulty determines the required probable prime chain length. E.g. if difficulty is 9.07233632, a probable prime chain of length 9 is required. The fractional part is used to determine how many length-9 chains are accepted. It scales logarithmically so that going from 9.8 to 9.9 is a lot harder than going from 9.1 to 9.2. The details for the fractional part are in Sunny King's paper.
The current algorithm also does more work for longer target chain lengths. That's why PPS numbers are also dropping.
Thanks Mikael, I read that it takes a 30 fold increase in processing power to increase difficulty by 1 . . . if so difficult to imagine difficultly ever climbing beyond 12 or 13.
999/(13*13) = 5.9 per block