PHA != side loaded apps. PHA == apps that they classify as potentially harmful (i.e., this survey is trying to put a number on how many devices have what they are classifying has potentially malware). This does not have a directly correlation reported for how many of those apps are side loaded or not.
I'd like to see some evidence of your claims.
actually google classifies all sideloaded apps as PHA
edit: so the number of sideloaded apps lies somewhere between 0 and the total amount of PHA