You mint a PoS block only if wallet is opened. But the reward of that block depends on coins age and their amount, that's all!
Send coins to your wallet, open it 1 day after and you'll get +0.0137%.
Open 2 days after instead and you will get 0.0274%.
Everything is in the source:
https://github.com/signatumd/source/blob/master/src/main.cpp#L1022
https://github.com/signatumd/source/blob/master/src/main.cpp#L1920