There are reports in the Google AdWords API Google Groups that the Google AdWords API is reporting inaccurate data from the usage tool.

First reports came in on March 19th, where an AdWords API user said:

In short, Google charges for certain level of usage for the API. AdWords API users keep track of their usage reporting to stay on top of costs. The usage reporting seems to not be updating correctly and costs are hard to control without that reporting.

Jeff Posnick from the Google AdWords API Team said:

Forum discussion at Google Groups.