A question about Carry-lookahead adder propagation delay
The delay calculated doesn't refer to the time taken to calculate the \$C_{out}\$ from the last stage. Instead it refers to the time taken for the full addition operation.
Time of Ripple Adder

To get an idea of what the symbol \$t_{x}\$ means, let's look at ripple adder first. Here \$t_{ripple}\$ refers to the time delay of the full ripple adder; i.e. the time taken to generate all bits \$S_i\$ and \$C_{out}\$. In the ripple counter, the total time is dictated by the time taken to calculate \$C_{out}\$, since \$S_{31}\$ is calculated just before \$C_{out}\$.
Time of CLA
However, in the case of carry look-ahead adder, \$C_{out}\$ gets calculated before \$S_{31}\$. Time to calculate \$C_{out}, t_{cout} = t_{pg} + t_{pg block} + (N/k)t_{AND-OR}\$ as expected. However \$S_{31}\$ is waiting for the ripple triggered by \$C_{27}\$. That rippling takes \$k\cdot t_{fa}\$. That along with time taken for generation of \$C_{27}\$ determines the delay of the full adder. \$C_{27}\$ is what introduces the number \$N/k   -  1\$.