The Steem payout is (relatively) fixed based on the votes you receive, but your SBD payout is tied to the value of Steem. With 50%/50% post rewards, you receive half in Steem Power and the other half is an SBD amount equivalent to the 50% value of Steem based on the assumption that SBD is worth 1 USD.

It is complicated, so your confusion is warranted.

And you're right. There is far less risk when you send a late bid. However, I know there are accounts out there that are keen on flagging people who use bid-bots last minute. I'm not sure why they do that... I don't see late-voting as abuse in any way.