Inconsistencies in ISO between Fuji and Olympus?

After over a decade of just using compacts and phone cameras, I am returning to "serious photography" camera systems. I've noticed things have advanced greatly over the past decade, and we currently have access to such wonderful equipment and at relatively lower price than 10-15 years ago..

After some deliberation, I decided to go with 2x systems :-D. For the less than the price of a new X-T4, I got 3x mint condition bodies instead - an X-T2, XT-20 and an Olympus EM1-MKii :-D.. - All such lovely bodies, with specific use advantages.. (Why I decided on these is a different discussion, and I'll start another thread if anyone is interested..)

But to my discussion.. After playing around with these cameras for a while I noticed something odd.. I noticed that on the Olympus camera, it would give roughly 1 F stop sensitivity advantage over the Fuji cameras..

Example - for a given shot, if I need f2 / 100thsec / ISO800 on the Fuji bodies, I can get the same shot at f2 / 100thsec / ISO400 on the EM1 (or 200thsec at ISO 800)

Now, I thought ISO standards should be universal right? So why the discrepancy? I don't believe there is anything wrong with my EM1 body..

If it is true that the Olympus sensor is significantly more sensitive than the Fuji, then when reviewers compare the 2x systems, they ought to compare, say, the Fuji's noise at ISO1600 to Olympus at ISO800.. which clearly gives performance advantage to the Olympus sensor..

Has anyone here using both systems notice this behaviour? I have confirmed this with the 2x Fuji bodies vs the EM1-ii across multiple lenses on each system..