Mike, the reason is:
The duty cycle is read backwards on the meter itself (On time, instead of off time). By reversing the leads you get the desired reading.
Key On engine Off : 70%, that is the reading you want before starting the engine.
Some people freak out because they get 68.9% or 71% any number close to 70% is OK. People cannot expect a lot of accuracy from an inexpensive meter.
Hope this helps.
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car