how is the labor burden calculated in the 6-1-2 reports?

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I am having a difficult time trying to figure out how the labor cost is being calculated on the 6-1-2, labor cost reports.  they are varying highly for my designer.  the same job had one days work at 31.80$ and another days at 75.20$, regular hours, not overtime.  I am wondering if it is calculating the labor cost based on the paycheck that the weeks time was on.  In other words on week of 12/11 she had $0 commission so her taxable burden was lower than week 12/18 where she received a considerable amount of commission raising her taxable burden higher. Any insight would be helpful.