The site makes it very clear, actually, that they adjust ticket sales in each year independently, based on the year that the tickets were sold. http://boxofficemojo.com/about/adjuster.htm goes into all of the details.

That being said, I don't know how much their calculations should be trusted, as they claim to use $7.35 as the ticket price for 2009 as well as 2010, but show Avatar's 562MM as adjusted up from 555MM starting in 2009 dollars.