There's a reason for that.
Let me explain this with an example:
1: Let's assume (for ease of computation) we'd have exactly 1440 masternodes.
2: there are (on average) 1440 new PIVX blocks per day, each worth 5 PIV, minus 10% for the PIVX budget is 4.5 PIV per day for masternodes and stakers
3: from 1: and 2: it's pretty clear that on average each masternode gets one reward per day
4: the amount each masternode will get depends on PIVXs seesaw algorithm: if there are a lot of stakers and few masternodes, the masternodes get more, if there are lot's of masternodes and few stakers the stakers get more. Right now stakers get 2.16 PIV per block, masternodes 2.34 PIV
5: since we know the exact number of masternodes (here: 1440) and therefore the number of rewards they get per day (here: 1) we can exactly compute how much each masternode will get per day, 2.34 PIV in this example.
6: but, we don't know how may people are staking, and which amounts of their PIV are used for staking. If only ONE single person would be staking he would get 1440 * 2.16 PIV per day, if there are 1440 people who stake (assuming they stake with the same number of coins, which they don't) each one would get 1 * 2.16 PIV per day.
Easy to see that because we have 3 variables (number of people staking, amount used for staking from each, and the time the wallet actually IS online and staking) it's not possible to predict how much PIV each staker will get per day.
Thanks for the excellent explanation!
I didn't read the white-paper but assumed already that the high reward had something to do with a low number of people staking. So when the number of people staking is low it can be more profitable to stake instead of running a master-node.
Are wallets (when they are online and staking) supporting the network in the same way as master-nodes do?