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Google is notorious for being very picky, having significant false positives, and not caring.

Most likely they aren't discarding these mail messages, but refusing to accept them in the first place.  This can happen easily when the sender doesn't have all that latest anti-spam technology turned on at their end, like DKIM, SPF, etc.

There are still a lot of old mail servers out there.  Google is a big organization and has the ability to jump on the latest technology quickly.  Small companies with their own mail server (for various reasons) don't know much about these things and don't want to spend time being mail server administrators, so just let them run.

For quite a while, mail from my domain to gmail would sometimes get to a recipient, sometimes not.  Mail to all other domains seemed to work normally.  Sometimes, but now always, I'd get a bounce message from gmail.  I had many things to do, so this didn't get much attention.  Real companies don't use gmail, so only message to a few select individuals weren't getting thru.  Most of the time, it was more to their advantage than mine to receive mail I sent.  I developed the attitude <i>"Screw this, you're the one who picked a bad mail service"</i>.

Eventually I looked into it and found that one of those settings (SPF ?) for my domain wasn't right or properly turned on.  After fixing that, gmail seems to be accepting all my mail now.

After the next new anti-spam technology emerges and gmail gets uppity again, this will probably happen all over.

So, the answer is this is probably due to lagging technology by the sender together with an overly-aggressive and holier-than-thou attitude by Google.  There is probably nothing you can do about it other than not use gmail.