Not necessarily different chemicals. Most of the aldehydes, esters, ketones etc used in ejuice flavorings are exactly the same across both industries. They just are soluble enough at say 1% (typically used in much lower quantities) in propylene glycol/glycerine. The inverse is true for the few water soluble ingredients used in low amounts in MCT-based or terpene-based flavors. It’s pretty common for ethanol to be used in store bought flavorings/nic-free eliquid bases to help give some leniency with solubility/shelf life.
The only ones that contain vastly different ingredients are the eliqiids with darker colors like Cola, Root Beer, energy drink flavors, etc. That’s also why you don’t really see these flavors sold in terpene mixtures or for oil-soluble flavors like in the cannabis industry. Some still try to offer them but they will turn dark brown or red and separate pretty bad.
The main reason ejuice tastes so different is the PG/VG enhances the sweetness and works overall better to deliver the flavor. Ejuice companies also go absolutely overboard on ingredients like ethyl maltol and diacetyl derivatives so they tend to be sickeningly sweet. Oils just don’t dissipate on our taste buds to come across the same way despite containing many, if not most of the exact same ingredients.