Manufacturers, knowingly or unknowingly, use commercial-grade PG when making cough syrups to cut costs.

i’d note that there’s zero technical reason why DEG would end up in PG. reaction of water with ethylene oxide gives you ethylene glycol, diethylene glycol and higher analogues and these are then separated by distillation. propylene glycol is made from propylene oxide instead, and it’s more expensive than ethylene oxide. diethylene glycol has little use on its own, at least compared to other glycols

however,

The physical properties of diethylene glycol make it an excellent counterfeit for pharmaceutical-grade glycerine (also called glycerol) or propylene glycol