Bid bots work like auctions. Let's say a bot has a 100% upvote worth $100, and 50 people bid for it. The vote worth will be shared among the bidders based on how much they bid.
Eg. A bot has a $100 upvote and 5 people make equal bids. The 5 will get an upvote worth $20 each.
Post promoters on the other hand work via multiplication. Let's say the bot has a return rate of 2.2x.
If you send $1, you'll get an upvote worth $2.2.
That's the major difference between the two.