You can not compare 2 different points cost models when one has 2 versions of which one can fly, and is a combat monster with heroic killing blow and a character mount to a mobile cannon with an armour save.
The Sphinx in whatever form has toughness 8, possible breath weapon, can have wounds healed back by their armies main lore and thunder stomp, they have base size in common, that is all.
How about next we compare the Arachnarok to a terror gheist?
The engine is metal lined and hard to get to not to mention the wheels have thick steel covers and rims to protect them, want to try and hack, through a few inches of steel?... Be my guest.
A Cannon is a wooden frame, a tank is a wood and metal frame, extra strength does not make things weaker.....
Armour is not the issue, the point is a cannon as standard is toughness 7, it does not suddenly downgrade because you give it an armour save.