thats because myrcene is actually called beta myrcene alpha myrcene is actually synthetic not found in nature.And easy experiment anyone can do...
Go get two of the same edibles.
Go eat a piece of fresh mango (high myrcene) then eat edible #1. Notice the effects.
Then the next day eat twice as many dried mangos (still high in myrcene) then eat edible #2 and notice the lack of effects.
Both are mangos, both are high in myrcene, yet it's only effective when the mangos are fresh.
Terpenes arise naturally from dehydration of terpenol geraniol. if you dried mangos the myrcene has been dehyrdated it changes into an analgesic pain killer. when terpines are distilled fresh they are still in acid form. they convert when drying. kind of like how cbda and thca turn into cbd and thc and cbn.