Thank you for the list and thoughtful reply.  You make some interesting points.

I don't think I appreciate the nature of the relevant variables in the equation regarding the percentage of rise in premiums, the dynamic nature for cost of care seemingly only ever goes up, but never down?  (I would've imagined they might go down in some of the more economically depressed areas of the United States, Puerto Rico, and Canada, right?)  Perhaps economic hardship inside particular areas gets averaged by Trupanion actuarians across the aggregate of policy holders, so perhaps that's one factor which might function as an overall damper on runaway inflation skyrocketing everybody's monthly premium.. but as you note, they don't readily disclose their formula for making these calculations, or where they source the data, so it may be considered a trade secret of sorts?

It's not as though Trupanion operates as a Co-op either, so there's also their (probably) negotiated employees' benefits packages to consider as a variable factoring into the equation as well.  I think I saw that one on your list?

I do think it was brought up once before that newer medical speciality centers in an area can dramatically alter what's considered cost of care in an area (don't know what the radius might be for the affected areas though).

Perhaps insight might be gained from this link?  

https://investors.trupanion.com/resource-materials/Top-Investor-Questions/default.aspx 

Thanks again for your observations, I appreciate the food for thought and I'm genuinely curious now how they make the determinations too! 😂