Allosaurus are found in packs and often mate-boosted. Any mate-boosted creature only takes 70% of the normal damage and therefore requires around 43% more damage and torpor to either kill or knock down. The alpha, which for the reason of being the highest leveled is the most sought after, has another bonus on top of this that when together with the mate boost makes them around 2 times as resistant to damage and the torpor it may come with.
Now stop crying about how it takes more resources to take one down and use your time to devise a plan to separate the Allo you want from the rest.
TIP: You are unlikely to outrun one on foot or with most carnivore mounts, so be sure to bring something fast or beefy.
if u are going for an allosaures tame the easiest thing you can do is lead one into a taming pen and then kill the others outside that way they do not have the alpha pack boost affect and you can easily and safely knock it out