whattomine calculations is way off. it is not giving accurate data when it comes to coin rewards. dont use it.
They are accurate, same as mine on
http://nullspam.ru/nodes/signatum/Reward is calculated based on average diff for the past 24h.
If diff spiked during the last hour and you are getting half the coins that doesn't mean calculations are wrong.
Difficulty is unpredictable, so any predictions for the next 24h will be even less accurate.