I don't think the US is exclusively generous, but your numbers are flat out wrong.
The 7 billion from Japan is the FY 03/04 number, while the 6.9 billion from the US is the 1997 number. It's comparing apples to oranges. Corrected numbers (from ODA site):
2003
United states $15.8 billion
Japan $8.9 billion
France $7.3 billion
Also from global issues.org: "Since 1992, Japan had been the largest donor of aid, in terms of raw dollars. That was until 2001 when the United States reclaimed that position, a year that also saw Japan's amount of aid drop by nearly 4 billion dollars "
On a little side note, it is interesting that in raw dollars, the US was second behind Japan during the entire Clinton administration.