I'm not sure where they got that number. Do they mean their success rate for cases they accept is 99% or the success rate for everyone applying is 99%. If the latter, I don't know how they would know. If the former, I am, as always, skeptical.
I don't really agree with that statement. If you are talking about success rate of immigration visas than it purely depends on the way you present your case. If you have all your documents genuine and available on time and if you have presented your file than chances of refusal for FSW cat isn't very high anywhere in the world.
It depends on how strong your case is. If you submit your file without having enough qualifying points, it's more than likely your application will be returned.
I guess your lawyer said that to give you some kind of assurance that your money will be well spent if you hire him/her. If I were you, I wouldn't count on it until I receive the first AOR. G'luck to you.