Charlie and I are learning about projectiles this term, the trebuchet and onsager's similarities and differences.
Even though they both chuck things into the air, that doesn't mean they're the same thing. One is bigger than the other so already they’re not the same. Another reason is the trebuchet uses weight to power it and the onager uses rope power to power it.
The trebuchet goes higher than the onager because the trebuchet has a higher arch. Quick fact the onager used to be used to smash castles to smithereens.