The 74HC595 datasheet specifies that this IC is a 8-stage serial shift registers with a storage register and 3-state outputs. The registers have separate clocks. contain an 8-bit serial-in, parallel-out shift register that feeds an 8-bit D-type storage register.
Data is shifted on the positive-going transitions of the shift register clock input (SHCP). The data in each register is transferred to the storage register on a positive-going transition of the storage register clock input (STCP). If both clocks are connected together, the shift register will always be one clock pulse ahead of the storage register.