I believe the primes are no longer in production. Probably those you can find on shelf are the left overs. Besides, the prime hd isnt too expensive. If you are planning for future upgrades a 26hd maybe a better option.
https://reefbuilders.com/2016/06/14/ai-prime-hd/
Iirc, the prime hd have additional led bulbs and 1 or 2 different colour bulbs as well.