More than anything, I think it's the suprise factor, in their being a difference.

Now I know about it, I can take steps to handle any potential problem & any questions regarding the differences.

So, thanks to Janet for highlighting the problem & George for his input.

Paul

P.S. George - my client can be very fussy, I have had discussions regarding the lack of accuracy in my figures & them want me to convert all of my calculations to be accurate to 3 dp. What we're talking about is 1/5th teaspoon of liquid in 200000 litres, when their precision of measurement is +/- 20 litres.