Value Area Calculations

I check two sources for the S&P emini Value Area each day. One is My Pivots and the Other is the Larry Levin (Trading Advantage) site. They send an email each day with the Value Area.

The two sources never agree and sometimes the numbers are radically different.

Examples:

Friday 09/03/2010 My Pivots VAH = 1085.50 VAL = 1082.00
Trading Advantage VAH = 1086.75 VAL = 1080.75

Thursday 09/02/2010 My Pivots VAH = 1082.75 VAL = 1072.75
Trading Advantage VAH = 1079.50 VAL = 1066.00

They are just "areas" and there are minor nuances to how different sources calculate the value areas, but I would still like to understand why the numbers are so different some times. Any comments will be appreciated.

Apart from the numbers that are posted on the forum there's also the Market Profile Calculator which might help.

The formula is here: Calculating Market Profile Area