I'm confused as to were the 50ng ml comes into play
The I/A carries a 50 ng limit because, being a non-selective methodology, it recognizes all 31 metabolite concentrations combined.

THC enters the body in its ingested form and exits the body as 31 different metabolite concentrations. The most prevalent form of these 31 metabolite concentrations is the THC-COOH metabolite concentration.

In order to test negative on the I/A, the whole combination of the 31 metabolite concentrations must register below 50 ng.

The GC/MS carries a lower (numerically) cutoff threshold at 15 ng because, being a selective and more accurate methodology, it recognizes only one metabolite concentration - the metabolite concentration that matters the most, which is the THC-COOH metabolite concentration.
In order for the urine sample to test negative on the GC/MS, only that one metabolite concentration (THC-COOH) must register below 15 ng.

Even though their cutoffs numerically differ, a 50 ng Immunoassay is functionally equivalent to the 15 ng GC/MS. The cutoff has to be set at a lower number in order to make the two types of tests functionally equivalent.