This is just an unverified claim.. without knowing his address, we can't really know - but the claim seems implausible...

Actually, the reward is calculated correctly for 50 % (actually more if you include possible compound interest) - but the intervals are growing (the coin-age of the average block getting reward goes still up.. - and so goes the network weight) - which means that you get your reward LATER (and might miss out on compound interest) - and that smaller wallets will have to wait even longer.
So if some people would actually take their wallet offline for a few days, this might help

(reducing network weight - but once they go online again, they will still get reward according to their coin-age - that has been building up offline... / that, of course, is not actually what is the intend of POS...)
Maybe I am too unlucky and I get smaller amounts. About the 80% maybe @adamd07 can show his calculaions or how he got that amount of intrest. About the offline acumulating coin age I saw many coins have this implemented so is not the end of the world. I wonder if dev wallet is staking or not...