Not all ads are served at the ad level. In other words, there are some ads that are served only at the ad group, campaign, etc. levels. The implications are that since not ads are included in the ad-level report, their associated spend, for example, won't be included at that grain. Therefore your spend totals may differ across the ad grain and another grain.
This is a reason why we have broken out the ad reporting packages into separate hierarchical end models (Ad, Ad Group, Campaign, and more). Because if we only used ad-level reports, we could be missing data.s