I think there is a lot of PoS currencies with fixed max supply (in my memory).
There are some, yes, you're right--I should have just left it at 'most' (I edited the original post to reflect this). QTUM is one example, although the max won't be reached for 31 years. OMG will eventually be another. Others, such as NEO and VEN/VET, have a fixed supply of the 'governance token' and then pay rewards in a separate 'utility token' whose supply may or may not be capped (e.g., GAS will reach a max supply of 100 mil. in ~22 years, whereas VeThor has no max supply). However, many PoS or DPoS coins have no fixed supply (e.g., XLM [not quite PoS, but similar outcome], Lisk, Ark, Strat, Nav, ETH [for now], PIVX, etc.). Note that many of the coins with no fixed maximum use other techniques, such as PIVX's fee burning/budget creation, to restrict the supply anyway.
Since several days, I watch the PIVX masternode reward with this website :
https://masternodes.online/currencies/PIVX/?convert=EUR.
.. and this reward is very curious : it can be $4 a day or $50 a day.
Is that a bug from the masternode.online website or it is the true ?
I know that the PIVX reward is adaptative between PoS and masternode, but here it is a lot (x12) in a very few time (in the same day) !!!
Can somebody explain to me please ?
I suspect that's an idiosyncrasy of that site. Generally, MNs reward at a very consistent rate--they are not subject to the same degree of 'luck'-based fluctuation as staking. You may find this site more reliable:
http://178.254.23.111/~pub/DN/DN_masternode_payments_stats.html