If you're using the 4d6 drop lowest method (default for 5e), the average roll (rounded) is 16, 14, 13, 12, 10, 9, with an average score of 12.24. The average score on the standard array is only 12, so 4d6 drop lowest is a little better. With point buy, you can get an average score of between 11.5 (15, 15, 15, 8, 8, 8) and 12.5 (13, 13, 13, 12, 12, 12), so it can actually be a little better than 4d6 drop lowest, but I think what you're seeing might be that players don't want to have close to the same score in every ability. They want to have at least one 15, for which they end up sacrificing more points.