Hey bit of a newb - can anyone explain this staking business?

I have about 20% of my wallet sitting in 'stake' and other 80% in 'spendable' and current 0 in 'unconfirmed'

I have received a small amount via PoS which state 'mined' (confirmed+accepted) + some which is 'generated but not accepted' (i.e. looks dead)

I go into advanced options in wallet (under send) and see in the tree view that the items which seem to align with the 'stake' bucket have a priority of highest' and are reasonably old ( 14-May ) and have 4073 confirmations.

The other 80% is also marked priority 'highest' but slightly newer (about 12 hours later ~15-May ) but with higher confirmations (6307)

I was expecting that after they matured (X-Days) they would move from 'spendable' => 'stake'

what makes them 'stake'-able? Please note that I do have multiple 'addresses' as I spread the incoming coins across multiple as I assumed (at the time) that if I added newer coins to existing address with older coins the 'clock' would restart.....

Somewhat confused......