Also, I assume there's a different demographic that uses Mail vs Outlook. Those different demographics will receive different types of email, which may or may not be represented differently by companies who use Litmus tracking which is how this data is being collected.

As far as I know and recall from the years I've been using Mail.app, it does not download external images by default.

right, neither does Evolution or Thunderbird. It's crazy that Mail.app does this.

Does Apple excluded Mail.app from their privacy focused strategy?

Things may have changed, but to my knowledge iDevices have traditionally been disproportionately represented in many metrics due to getting heavier usage from their owners. Android wins by far in sheer units sold and in use, but iOS users use their devices so much more heavily and frequently that the average iOS user has a larger usage footprint than their Android counterpart.

Outlook on Mac consumes outrageous amounts of ram...

No - I still don't like it myself but they've made some pretty great strides in feature parity and have excellent integration if you're a Teams shop.

its gotten better but its still not great.

Yes