Main reason for me is that I have bought humble bundles, donated to gamejams, and gotten keys off of legit and grey-market sites in the past in conjunction with buying directly from Steam. Those aren't included in the Steam spend category.

You should look at the External Funds thing mentioned in the article. It gives a different spending breakdown than Purchase History.

Help > Steam support > My account > Data related to your Steam account > External funds used. (Its the 13th item on a huge page full of stuff.)

This isn't a 3rd party tool, it's a separate Steam Support page that lists your total purchases. It basically takes the data from the Purchase History section (assuming that you usually pay directly and not using Steam gift cards) and totals it so that you don't have to do that manually.

It's been known for atleast 7 years: https://www.reddit.com/r/GlobalOffensive/comments/8m9fxm/what_is_difference_between_totalspend_and_oldspend/

edit: no need to point out those are 2 different links, ill leave this up

To judge my friends, of course.