Hypothetically- A sweet stout and a red ale both get 45 points. How is the winner decided? It's very difficult when comparing beers of different styles to determine which is "best", you're comparing apples and oranges, to use an overused phrase. The reason why there are categories is because each beer has one set of guidelines to aim at, which ensures ease of judging.
This already happens with a BOS show round anyway though, it comes down the overall impression the judges have of the beer.
There are also countback procedures in place, assuming the comp is based on BJCP, or they can develop their own. Flavour and overall impression are generally the most common.