TLDR - The use of notation is poor - I'd call it sloppy. His post would be much better (to me) without the example.

Most alarmingly he's written an inequality as an equality, and he's not really explaining the units / percentages at all. I think this type of thing is really unhelpful to people who struggle with maths and maths notation. It seems evident that the last reply's author does struggle and the example is part of their confusion, it certainly didn't help.

It makes me think he doesn't give a shit what he writes or about the audience. That makes me inclined to give no shits about it too. Either way this style of communication is all red flags to me - I guess thats why I avoid all this twittery in general.

Anyway, I guess I care enough to type a few more shits . . . I'm sure i've already put in way more thought than the OOP . . . I think he should have written something that spells out a bit more what is going on , and used an inequality, like:

• 100 - 100 x (10/100) != 90 + 90 x (10/100)
• 100 - 100 x (10/100) > 90 + 90 x (10/100)

Or:

• 100 - 100 x (10/100) = 90
• 90 + 90 x (10/100) = 99

As others have pointed out, in some contexts (like price indices) people convert time series data into indexes where 1 unit is referenced to: 100*1/(nominal reference price)

So for units defined as "percent of reference price", we d can use the simple expressions:

• 100 - 10 =90
• 90 + 10=100 (edited my own slop)

Some people might, also sloppily, refer to the 10 as a "percent change" as shorthand for the units. It is a percentage, but one should be clear percent of what.

If the context is more specifically weighted price indices then it can get more complex, but commentators may simply by saying "up by 10%", when it's really a more complex index value movement where the weightings can change. This all makes financial data in particular very confusing to those who struggle with maths, which is bad and it'd be nice if these things were communicated in a way that makes it as easy as possible for them.