How does primary slots and secondary vrf slots work?

Question

I am struggling to understand some behaviour on a custom 7 validators solochain.

Currently using babe with a 1/4 primary slots probability, the block time is 20 sec, and we have PrimaryAndSecondaryPlainSlots as allowed slots.

Things i'm wondering :

• While adding log the babe module, I computed a real primary probability of arround 22.5% instead of 25% (on a "relatively small" sample of 10000 blocks).
• Also on the network, I notice a lot of fork / uncles set.
• The most problematic is this morning, I saw that a slot was missed (both primary and secondary) and the time between two block was twice what it should be (40 seconds instead of 20)

Is this a normal behaviour ? Is there an actual probability of missing a slot to produce a block ?

Usefull links :

• Config
• Network explorer
• Block - Parent block
• Logs