In the AMBA APB documentation, we find that the master start write transfer (data1 in PWDATA) when we are in setup state ( PSEL=0 and PENABLE=1).

https://www.dropbox.com/s/qp3id0hnzbt8l03/AMBA%20APB%20PREAD.PNG

Does the slave register's value change in the enable state or in the setup one?
In the other side, when we have read transfer, data1 in PRDATA start only when we are in the enable state (PSEL=1 and PENABLE=1).

https://www.dropbox.com/s/icw75ivqx9kq2tr/amba.png

APB writes can happen in the setup-phase [i.e when PSEL & PWRITE = 1]
there is no need to wait for PENABLE because you loose one cycle for write.
it depends on how the slave responds to the write transaction.

APN read case - in the diagram slave is responding in the next cycle when PSEL & ~PWRITE.
Its not waiting for PENABLE, hence gaining one cycle.
in this PREADY & PENABLE go high in the same cycle.

if you put PRDATA after PENABLE then PREADY is high the next cycle.

Hope this clarifies the question.