peercoin-0.8.5

I installed peercoin (linux-64) to a new PC. I imported private key with "importprivkey". Once sync was finished, I was surprised to find that I have more coins than I expected (QT client). I verified with "getbalance", it reported the same value. I expected to have about 55 coins but 77 coins were reported. I listed transactions (listtransactions) but there was no evidence for extra coins. I rescaned blockchain (rescanblockchain) and balance was fixed, to 55 coins. Difference was 17.400945 PPC, these 17 coins were reported but were not in the wallet.. I don't remember I ever spent a single PPC coin from this wallet. Do you have an idea, what was source of these illusive coins?