There are different ways of taking an average depending on the distribution of the data.
The common method of summing the values and dividing by the number of data points requires a reasonably even distribution of values. If outliers, values very different from the rest of the distribution, are included, that has the effect of skewing the result and providing an inaccurate average.
An alternative approach might be to take the median. That finds the middle value of a range of data. Another method is to use the mode, which is the most popular set of values. That ignores outliers and is useful for an uneven set.
Of course, in order to produce anything meaningful the criteria needs to be clear. If someone is using goals scored as the criteria, that would produce largely meaningless results unless the group only consists of attackers. When we make an informal assessment, we are often vague about the criteria using our impressions rather than a measurable quantity.
Football is rife with inaccuracies which of course makes outcomes unpredictable. That is a major contributor to the excitement we fans experience.
