This may not be accurate in all cases.. and as jon suggested its based on an alorithm based on color contrasts. Theres one another tool which does the same thing called Feng-GUI * you can only test once in 12 hrs...
If you want exact heatmap stats you will have to go for paid options like crazyegge, clickheat, clicktale etc...
Please find attached a screenshot comparing the two tools. In both cases the results are almost identical bar the logo... which means the algirthm is more or less the same.
