[verga]

it appears that the animals used for sacrifices weren’t kept by the temple but were brought there by merchants who sold them to individuals following the law.

the shepherds were part of the lowest memebers of society. the merchants were somewhat higher. It would make sense that the merchants hired them to tend those same flocks.

[Varda]

Traditionally flocks were owned by a family and the job of shepherd was given to a family member but what you say is possible. However, it doesn’t follow that the flocks were there year round in an area doesn’t support year round grazing.