How can it keep charged in use, but be unable to recharge?

I spent an hour on the Android phone with a friend, and because we
kept getting dicsonnected on whatsapp and also on Skype, the phone's
screen was on all the time, but I was plugged into a small cubicle
charger the whole time, and the phone charge stayed at 77% the whole
time.

Then the phone call ended, I closed the light, but left the phone on,
and let it finish charging for an hour. But after an hour or more,
it was still at 77%.

How can that be?

Later I was able to recharge the battery fully, use some and recharge
over and over up until now. Using a different cable. But if the
cable were bad, how to account for the first paragraph??