I found an article for you that may help explain about the JAK2 gene and how an acquired mutation to this gene can cause issues within the bone marrow.
Essential Thrombocythemia (ET) is just one of several conditions that can be related to the JAK2 mutation and are referred to as myeloproliferative neoplasms (MPNs)

MPNs occur when blood stem cells produce too many of one or more types of blood cells, including red blood cells, white blood cells, and platelets.
This is generally a progressive condition and many people diagnosed will be in an active surveillance period until there are significant enough changes to warrant treatment. And sometimes there are no progressions and treatment isn’t necessary. Everyone is different! That’s why you’ll see some members who are not on any medications and some are taking increased dosages.

Not everyone who has the JAK2 mutation requires treatment and conversely there can be people with elevated blood cells who don’t have this mutation. The body, especially the blood/bone marrow/immune system, is so incredibly complex as you’re finding out!
From Verywellhealth.com:
https://www.verywellhealth.com/jak2-mutation-5217909
Hopefully this article will help to clarify some of your questions. If not, let me know!

Hi, I'm 68. Just diagnosed with Jak2. They did a DNA blood test. My platelets are fine.