Like any sport with judges, it will always be somewhat subjective. Of course, there are guidelines, which the comment above goers over quite nicely.

In boxing, each round is scored by three judges. They can give up to 10 points to each boxer, but the overwhelming majority of the time, the score is either 10-9 or 10-8.

At the end of the match, each judge totals up their scores to decide who they think the winner is, and whoever is declared winner by at least two of the judges wins the match.