In some cases you can see a difference between the total costs reported in your campaigns report versus the total costs reported in the keywords report.
The keywords report only shows the results of the keywords performance in Google Search. If you've selected "Display Network" when you've published the campaign (available in expert mode), those costs will not be reflected in the keyword performance report as Google only reports the totals for the Display Network and not per keyword.
However, the total costs will be calculated in the campaign performance report.
If you feel this is confusing or not adding any value to your results, you can also create a campaign without the Display Network activated. By default Display Network is turned off.