The 50-50 ratio is a rule of thumb, but most hatchers will tell you that it can go to the extremes in either direction. Some folks luck out with great pullet vibes, others find that they get bunches of boys!
There are also theories that temperature plays a roll: that higher temps during incubation yeild more boys. This temp theory is true for reptiles, I believe, but not so for chickens. The sex is determined before development. Others swear by this, but it might be that the male embryos are simply better able to survive the higher temps, not that it "makes" more boys. None of this is proven, though.
There is also talk about egg shape being a factor, but these really seem to be wives tales. They could make for some fun test hatches, though!
Your hatch rate can also depend upon where you get your eggs from. As a rule, a 50% hatch rate is good for shipped eggs, though many do FAR better than that and many others worse.