They use what's called a dual line which means that the odds you see are not the odds you get, if they think you may be a winning player.
If you show preference for dogs, say, or any type of soft line by them, they will pull a switcheroo on you and change the line in the last minute before you bet. This is totally unethical and unacceptable in my opinion. If someone is capable of one type of unethical conduct they are capable of other types too, so I refuse to do business with them.