Mechanism of Biosynthesis of Terpenes

There is a specific part of the biosynthesis of terpenes that makes me confused:


[![enter image description here](https://i.stack.imgur.com/C9B9m.png)](https://i.stack.imgur.com/C9B9m.png)


According to this mechanism, the double bond of the isopentenyl pyrophosphate attacks the primary carbocation, when it would make more sense to attack the tertiary carbocation that could be formed by resonance. Am I missing something here? Clearly, this is not a concerted process if the mechanism is displayed in this way, and an intramolecular resonance conversion would be faster than a nucleophilic attack. 


