Basically someone with Auditory Processing Disorder has no actual problem with their ears, they can have A1 hearing, but the problem is that their brain doesn't process the sound received by the ears. This is particularly the case with the sound of the voice.
In hindsight I can see that I have this all my life. It explains why I often hear someone talking but don't react to what they say, it explains why in loud enviroments I cannot hear what people say to me (lipreading is a skill I value massively), and it explains why I find it hard to follow long strings of verbal instructions.
If you would like more information on this APDUK.ORG are an excellent source of information.