Every post broadcasted to the steem network has a comment option available named allow_curation_rewards. It's true by default and gives %25 share of the post rewards to the curators.
If the original author set this flag as False, this reward goes to authors as an extra.
Busy implemented this this by default recently. However, steem.rocks' potential reward page wasn't taking into account that flag.
This bug is fixed with the commit 28aad36e.
Thanks @espoem for reporting the bug.