Why is the tractive effort for steam locomotives calculated with different steam pressure
constant?

85% of max working pressure for rod locomotives
75% of max working pressure for geared locomotives (Shay, Climax, Heisler, Willamette)

Why and when was that made and who made it? Any idea?

It is confusing to try to compete two of them for their TE.
e.g. try to compare (both have 200lbs rated boilers)
Baldwin Mallet 2-6-6-2T "16 28/46 1/4 DD" rated 37.545lbs@85%
Lima Pacific Coast Shay "3-PC-13" rated 38.184lbs@75%

I know that Heisler finally got wise and went to 85% in their catalogs and in doing so,
they upped the t.e. on the 90 ton model of their West Coast Special Heisler from
around 43,000 and change up to around 49,000 lbs.

That's a hell of a woods engine. Of course, you still had to get your train started.

You're right, it would have been a lot easier for customers to have a common
denominator for everything when looking for new power. Salesmen were expected
to give tonnage ratings though, when out in the field. so the playing field could
have leveled somewhat.