Once upon a time there was a writing competition which nobody won. The entrants were offended: They could not have been that bad, could they?
Sadly, it turns out that they were, possibly, not that bad. It was simply that many of them didn't obey the rules, didn't, for instance, put their names on their entries. Such a small mistake to make with such tragic ramifications: it meant no one read their story.
What this means for you and me is that, simply by following the rules of a competition, or even the rules for submission to any anthology or zine, you're ahead of the game. If we care enough to get it right, to make it legible, then judges and editors will care enough to try reading it.
Then come the other mistakes we might make. Personally, I think many of them might be made in the haste of trying to get the entry in on time, but at least it's pracice and practice is good for us.
Here's a link to a post on the subject. It's old, but it's a good one, and it does help you sympathise with the judges of short story competitions.