worn brake pad warning light

[A8 Owner]

Well other day this light came on. So new pads ordered, got them for steve and mate to put them on. Both pleased as it was the later style, so just spring to take off and pull back pistons. Well all went well all changed. Wow cant get over how much pad there is left even tho light came on, what a waste.
Any how all put together was told to go up road and test and get brakes working/bedded.
Got in car as soon as i put it in gear light comes back up. They re took the wheels off to double check all was fine.
Can anyone help .Why it should be still on.
Does it need to be plugged into computer to reset it.???