Why are the values not always conclusive?
This is a very good question. Unfortunately we do not have a final answer for you. For our evaluations within the Toolbox we use the data that Google sends us through the Search Console API. However, if you compare the total impressions to the sum of impressions for individual URLs or keywords, you will find noticeable differences (even for small domains, where all their URLs and keywords are listed). One possible explanation could be that Google has set up specific minimum limits for releasing data which then get applied differently to different data.