A temple priest has more knowledge about the ritual of worship, poojas, havans and yagyas than about Jyotish. Jyotish forms only a very small but integral part of his work. This is why he is not likely to have proper and deep knowledge about Jyotish.

On the other hand, an astrologer or a Jyotishi studies only Jyotish and strives to go deeper into this science. His focus is mainly on the mathematical and predictive part of Jyotish and not on the ritualistic religious methods. 

Jyotishi is an expert in Jyotish whereas a temple priest is an expert in religious rituals.