The reason is that when it’s on, it may not ever get to 100%. As it is always a bit busy. DRAM constantly needs to be refreshed, to not lose its contents, for example. And the radio module constantly needs to amplify incoming EM waves and interpret them, in case some call or message may be received. So the battery is constantly trickle-drained, while being trickle-charged too. So it can take way longer, and be bad for the battery. But that’s only with pretty bad charging implementations that can’t bypass the battery. Nowadays, it should not be a problem. At least I hope so. (My information is based on laptops, where a decade or more ago, you had to take out the battery when on AC, to prevent slowly damaging it.)

6 hours? So the battery was already pretty much dead, no? And you tried to fix it, because you thought it was just a glitch. Unfortunately is wasn’t, your battery was already down to 3 hours, and this simply told the OS about it, that was living a lie… ;)
That’d be my guess.
You are right though that this should never be done unless you know you need to, as it does damage the battery a bit. But usually it’s for a purpose that is worth more. In your case (a battery on its last legs) it definitely wasn’t.

But hey, batteries cost like 10€ on eBay (don’t support evil Bezos from planet Bezos ;), are usually the easiest component to replace, and you’ll have a proper runtime again. (6 Hours is about what a 1999 notebook got, so really not enough to use your phone.) With a bit of luck there’s also a larger battery for it.

Yes, "recalibration" is just measuring the "empty" (discharging more would damage it more than is reasonable) and "full" (charging more would damage it more than is reasonable) points as given by the battery controller, and how much is there in-between.

You can recalibrate as often as you like. It's just that it will cause a tiny bit of damage each time, cause it will really bring the battery to the liimt, but for above good purpose.

In your case though, I think you got a bad battery. (Remember that batteries got maybe manufactured years ago, and batteries degrade when just lying on the shelf too! So a brand-new, never-used, battery that's bad is entirely possible. But many sellers don't even know that, and sell it as new without mentioning the year of manufacture. Or it can just be a bad manufacturer.)

If the battery shutdown percentage gets worse and worse each time (especially each calibration), you definitely got a bad battery, and should demand a refund. It's not OK to sell you their old stock as new.

Hier auf KEINEN Fall ein Werkzeug aus Metall (wie den iSesame) verwenden!

Die Funken die dabei sprühen sollten sehr klar machen, daß das keine Gute Idee für den Akku ist. XD