It means you will get from that contract that value in 1 year. ( but I don't think this is good calculated )
I think its safe calculations and most like your return going to be better, but no one is going to complain for it. If you get less = big drama !
the only way to get less is end of mining.
Theoretic calculation:
0.00100427 BTC per day
0.00702989 BTC per week ( they say 0.00702992 BTC so who knows )
0.02811956 BTC per month
0.33743472 BTC per year
1.6871736 BTC per 5 years
Its all "correct" but if page say 0.33743472 return first year and ppl get 0.31743472 return you can expect much drama how they fraud. Posting less estimated return and get more in real going to make ppl happy. So its smarter this way.