How can these bikes have similar stack?

The first is an Enigma Echo.

See the size 55. ETT is 55, HT is 16.5, and the stack is 565

The second is a Jamis Renegade

 

See the size 54. ETT is 55, HT is 13.2, and the stack is 565.

So both have the same stack but with a very different head tube, The BB drop difference is about 3mms only.