The C's do not have a +13 rating with Amir on the court. That is probably some junk pulled off of basketball reference, the actual net rating is +1.8. Amir has been terrible.
Well, it looks like it is from Basketball Reference.
But I'm not sure why you call it a junk stat.
What is means is that for every 100 possessions Amir is on the court the Celtics score 120 points and allow 107 points, thus the +13 number.
It certainly isn't the end all be all of stats, nor does it necessarily mean Amir is actually good at basketball.