I'm not so sure that it's a case that selenium is present in...

I'm not so sure that it's a case that selenium is present in some ores as opposed to others as such, but rather the leachability of the ore.

Selenium is used in electrolytic manganese metal (EMM) production for non-battery applications since it makes the production process cheaper. Given its not desirable for batteries that makes the electrolysis process more difficult (expensive?) but might explain the increase on day 2 if they have used a small amount of selenium in the process in order to achieve 99.95%.

So really its the ability to skip the EMM step (that appears to require selenium) that gives E25 a benefit.
Can this be replicated by other producers? It is unclear but it does seem E25 have a strategic benefit if their claims are to be believed.

A good (essential) read:
Some manganese is more equal than others | LinkedIn

Disclaimer: I'm a smooth brain, @PirataCzornomaz can probably validate or correct me.