I think some coin developers are definitely competing in PoS reward rates, yes. The majority seem to have to have found a sweet spot of about 7-8% ROI a year, but some PoS coins give a lot more in return, whereas others reward with a lower amount. I do believe the higher amounts are simply higher to lure investors to pick them up. I'm not sure if it's a long term strategy that will continue to work for those coins.