Hi @krosebraugh,
If it only happens occasionally, it could be due to one or more problems periodically stopping the import from succeeding.
Sometimes it may be that the hosting environment doesn’t afford scheduled tasks (like saved recurring imports) enough time to complete; other times it may be connectivity issues between the server hosting your website and the source calendar.
If in the main it works, though, and regular imports continue, it’s probably safe not to worry much about this.
(Apologies, incidentally, for the delayed response – unfortunately this fell through the cracks 🙁 )