gross profit margin

I suggest using some simple numbers to illustrate: Say revenue should be 120, cost of sales is 100, so GP is 20 i.e. GP% is 20%. Say revenue is overstated at 130 .......... GP% is 30%.

I am confused by this. I thought Gross Profit was GP/revenue so 16.67% or 23.08%

@brucen - you are quite correct - but the argument remains the same whether you consider markup on cost of sales (my calculation) or margin on revenue (your calculation) - if revenue is overstated, the % change in profit (however measured) will be overstated (because cost of sales is unchanged). Hope that makes sense.