PageDepth 1 and PreviousPagePath entrance are not the same, reporting on metrics of landing pages
I'm trying to build reporting based on landing pages only. You would think I could use PageDepth==1 for this or PreviousPagePath==(entrance) and that these two dimensions would yield similar results.
Below you can see a case where this is not true by a large margin. Does someone have pointers for me what causes PreviousPagePath to be set to (entrance) while it is appearantly not the first page in the session? I tried to reproduce this by visiting the website via direct and re-entering the website through organic. That 2nd pageview-hit did actually had as PreviousPagePath (entrance), but so was PageDepth set to 1 (and not 2).
PageDepth is also reset by the session time out (30 minutes), isn't it?
This data is from a Classic GA account.
The specific 404 pages I was looking for when filtering for "entrance" are clearly 404 pages that currently are only accessible through organic results. But when filtering for PageDepth==1, I get only 70% of the the results.