Why do the common size statements not balance?

If the sample size is less than 30 in our industry data report, the common size statement may not balance vertically. When the sample size is less than 30 we look at each metric separately and will throw out any metric that falls outside of 2 standard deviations of the mean. This can cause the common size statement to not balance, but provide a more reliable and non-skewed percentage for each metric.