Pretty much every brew a person makes isn't 100% adherent to any style. There are glaring deviations from the guidelines and relatively minor ones. I think that minor colour deviations are probably the most common issues, with major colour deviations being quite common as well. Next comes hop character/bitterness/hop variety, then gravity.
When you sit down and fill out the competition's entry form you generally have to balance what you tried to make with what you actually ended up with. So if your aim was to make a 1.038 mild but your efficiency jumped for some reason and you ended up with 1.043.....whoops. Do those 5 points make a huge difference? Not really. Is it still a mild? Probably. Is it fair to enter it as a mild? Sure. Why not?
We all take our chances when we enter beers in a competition. Will the judges pick up on the fact that your gravity was 5 points too high? Probably not. If it was 15 or 20 points too high they probably would. But every other entrant probably did something similar with one of their entries. "It's not quite an x, but yet it isn't really a y....What should I enter it as?"
My club's most prolific (medal-wise) brewer has a saying: "A successful brewer can't be proud." In other words, feel free to change the category if the beer turned out different from what you were planning. If you were aiming for a Dortmunder Export and the beer turned out too malty sweet, enter it as a Munich Helles instead. If it had instead turned out too bitter, enter it as a German Pilsner. Just keep in mind that brewing isn't an exact science; for confirmation of this just get 10 people to brew the same beer using identical ingredients - 10 totally different beers will result. Hell, I can't turn out exactly the same beer twice in a row. And the BJCP style guidelines aren't set in stone, either. They change along with changing tastes. They're just that, guidelines, not hard and fast rules. And they're subject to the most fallible instrument in the world: human perception.
I guess I should keep quiet about the wit I entered as a stout last year just so that I wouldn't have to judge that category. You'll be relieved to know that it didn't win any medals. B)