Maybe she just wanted revenge on her SO by making an offer. It may have been free. There are several reasons to pass it up like she's fugly, may have something extra to give you that you won't like or you have no protection, or other reasons if you're more picky like she is married.
People will complain about others and then do the same thing or something similar but in their own mind, it won't be the same. I wouldn't try to figure it out. It's like people's definition of sex. Some people have rationalized that it's only sex if things are going on down near the waist. If she's only sucking, then that's ok and not sex as some people call it. Some people have a difference of opinion on that one.