I would say, then, that all they have done is check for power, and not polarity, i.e where the positive and negative are connected -a lamp will work either way, a multimeter will give a negative reading if it's the wrong way round.
Get a meter, stick the red wire into the middle, and touch the outer case of the socket with the black wire, have the meter set to 20V and if you get a reading and it's negative then you have it wired the wrong way.
Lets just hope the chargers you have tried are reverse polarity protected -otherwise you could have fried them.....