It's very simple, if they are seeing 14+ volts with everything on the secs is misreading, the alternator won't give out more than it's set to in the regulator(at time of manufacture).
A really good reading with everything on and the engine idling would be 13.4, and that would be really good, more likely to be 12.8- 13.0.
You could try another wire between the alternator terminal and the battery +, as this will have built up much resistance over the years.
tabetha