My sister admitted to the correction and from then on made the tea according to specifications.

TIL: When talking about tea, Brits sound like us Germans. ;-)

I had this arguement with an international student at university.

The thing is when you're boiling water in the kettle you're heating all of the water to a very high temperature. When you boil it in a microwave only the water molecules near the surface are actually getting heated because they absorb the microwave energy which essentially blocks other water molecules from receiving any energy, also the cup will also absorb a lot of the microwave energy. So the only water molecules getting heated are the ones at the surface.

So in a kettle you boil bottom up which agitates the water at the bottom causing it to bubble up to the top allowing more water to be boiled at the bottom, in a microwave your boiling the water top down, which means the water at the top boils and often then just fucks off into the rest of the microwave, so you get uneven heating and therefore it tastes weird.

If you boil water in a glass cup in a microwave the glass doesn't absorb the microwave radiation as much, and you get better heating, but that tends to break the cup.

There is a reason no one boils water in a microwave if they have a kettle. It tastes awful.

Your sister's test had a pretty high α-value. It's not surprising your tale is being told.

Curious now how (if?) she makes iced tea. Steep it in cold water in the fridge or boil the water, steep the tea, then refrigerate.