My guess is that they are using the older coin-age style proof of stake protocol. While coin-age systems do allow your coins to gain age that is then rewarded by stakes when you turn the wallet back on, it has some significant stability issues. I worked on fixing one case where a large number of old coins coming online in a coin-age coin caused the entire network to roll fork and roll back several hundred blocks. I do not know if that is the system they are using or not, but that is my guess.
JJ12880