Ok how can I explain it hmmm
Let me explain it with an example. Tipu gives out 100% of the sbd it gets for the upvotes. Lets asume there is only one who delegates and that tipu upvotes for 200% of the sended sbd:
Poeple are buying upvotes for 100sbd for that day and tipu upvotes for 200$ until it reaches the vote% limit and cant upvote anymore.The delegator gets 100% of the sbd so he gets 100sbd.
If the steem prize rises so does the vote amount do, so that tipu can upvote more until it reaches the vote% limit and cant upvote anymore. The more tipu can vote the more sbd will be earned.

You dont get a fixed apr for a delegation time, it just depends on the steem prize and the amount tipu upvotes for.