A Google AdWords API Forum thread has complaints from one API user that the data for the monthly keyword volume looks wrong. In response to his complaint, Googler Eric Koleda explained how recent that data is.

Eric said:

Now this question specifically answers that API calls and not specific to the keyword tool. But I suspect both the API and the keyword tool are about equal in the time they get a revised data dump.

So just know, that checking new keyword volume in the first week of the new month might not be getting you the results you expect.

Forum discussion at Google AdWords API Forum.