It was discovered within the source data that a single Ad can be associated with multiple ad groups on any given day. Because of this, it was determined that the is_most_recent_record logic within the stg_google_ads__ad_history model needed to account for the ad_group_id as well as the individual ad_id. As a result, the most recent record of an ad could possibly contain a unique combination of the ad_id and the ad_group_id.
This logic was only applied to the stg_google_ads__ad_history model as it was discovered this relationship was unique to ads and ad groups. If you experience this relationship among any of the other ad hierarchies, please open and issue and we can continue the discussion!
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.