weird calculation issue

Hi,

Unfortunately I don't have MS Excel installed in my laptop so the below was created in openoffice. I wasn't sure if people who do not use openoffice would have been been able to open the file so I'm attaching a screenshot of if as it's quite simple really.

The attached screenshot illustrates a 2 year comparison for 2 products (Product 1 & Product 2), in terms of quantity, total cost and unit price (total cost / quantity). In row 5 I'm calculating the combined quantity, total cost and unit price for each each year.

Lastly, column H shows the variance in the unit price for product 1 & 2 and the combined too. It's the combined that confuses the hell out of me basically..Why would the combined unit price variance be -93 when the unit price variance for product 1 is -33 and for product 2 is +466? Shouldn't the combined unit price variance fall between these two numbers??

Any ideas??

Thanks everyone

Dimiliv