As Hawkster already stated, the star system isn't meant to be definitive for everything. A quest like Liberating Dahshur encourages you to snipe buildings ASAP with wide open bases (at least in co-op), benefitting cavalry that can quickly take out production buildings, while a quest like Valley of the Kings encourages you to slowly wipe out the enemy with pallintonons and other ranged siege, due to the sheer number of fortresses, towers, and production on elevated terrain. Trying to purely use raiding cavalry on valley of the kings will not go very well, with no way to quickly stop the flow of enemies, while using pallintonons on dahshur would probably be quite slow.
Also, star rating is based on the optionals being completed, and can also be subjective based on what strategy a player may favor. It is why many times, quest star ratings get changed. For some repeatables, not enough people may play them to provide an accurate consensus for them.