There are some reports via Google Webmaster Help forums that the Webmaster Tools API results do not match up always with the Webmaster Tools web console.
One developer noticed the differences and posted a question in the forums asking:
I'm using the Google Webmaster Tools API to get data about our site's keywords (in Google's eyes).It's great.
But something's strange- the keywords showing up in the API feed are not quite the same as what's showing up when we used the web-based tool.
Has anybody else noticed this difference in the data?
I know that the web-based data is quite a few days behind, so one theory I have is that the API data is fresher. Just guessing at this point.
This is not uncommon being that the data is refreshed often and data filters can play a role in the results. But also, sometimes, APIs access slightly different data sources and can throw the numbers off. Finally, Webmaster Tools is known to have reporting bugs often - so that can play a factor also.
Or maybe, this was just a random fluke? Do you have this as an issue also?
Forum discussion at Google Webmaster Help.