Revolutionary War: All figures from the Revolutionary War are rounded estimates. Commonly cited casualty figures provided by the Department of Defense are 4,435 killed and 6,188 wounded, although the original government report that generated these numbers warned that the totals were incomplete and far too low.[18] Nevertheless, the numbers are often repeated without this warning, such as on the United States Department of Veteran Affairs website.[19] In 1974, historian Howard Peckham and a team of researchers came up with a total of 6,824 killed in action and 8,445 wounded. Because of incomplete records, Peckham estimated that this new total number of killed in action was still about 1,000 too low.[20] Military historian John Shy subsequently estimated the total killed in action at 8,000, and argued that the number of wounded was probably far higher, about 25,000.[21] The "other" deaths are primarily from disease, including prisoners who died on British prison ships.
This helps explain the disparate numbers I have seen over the years. As you likely know, 11,000 patriots were estimated to have died on the prison ships in NY Harbor alone.
In other words, the best they can do is speculate.
I did find this chart:
http://en.wikipedia.org/wiki/United_States_casualties_of_war