You have identified a discrepancy, but once again this is a case where the context is all important.
Given that par means average, the goal in golf is to finish using the least number of strokes, so in this case below par is a good result.
If we were talking about soccer, and par was the average number of goals that a team has scored in each of their games, then below par would be a poor result and above par would be a good result.
If we are to generalise and assume that most people would use an ascending scoring system of measurement such as percentages where 100% represents full marks, then below par would be considered a poor result.
However as discussed above golf uses an inverse scoring system where lower scores represent better performance, and therefore in this case below par is a good result.