In these examples the instruction execution time was buried in the read and fetch cycles. A JMP, POP, ADD, and most other operations are quite simple. Others are not. Execution time is a function of the operands supplied. Here's where adding transistors to a device improves performance.
Fetches read instructions - always from memory.
The difference is subtle. Sometimes, as on the Z80, the exact timing may differ a bit between the two. As the industry evolves, though, the difference in timing and signals is disappearing. The hardware doesn't care or need to know what's going on. An address comes out, the CPU asserts the read signal, and the selected memory device transfers data back.