I'm not having a problem, rather this is something I've been curious about for a long time. I shoot on a Canon 7D at 24fps. When I slate a shot, I notice that the audible "crack" of the slate happens one frame before the arm makes contact with the body of the slate, no exception. This is the audio that's recorded with the video directly on my 7D. Does this happen with all cameras shooting 24fps? Does it happen for a reason?
Where are you checking this and no it doesn't happen with all cameras but that said- DSLRs are not always perfect sync audio cameras in video and a single frame offset is not unheard of. My suggestion to improve audio is to record double system sound with a nice mixer and outboard audio recorder and sync in post. Some NLEs like FCPX have automatic syncing and you can get third party apps to do this as well such as http://www.singularsoftware.com/pluraleyes.html