in PCI, CBE#[3:0] can be any combinational, such as 0100 is valid. However, in AHB, there is no byte enable at all. therefore, PCI can write 3 bytes in 1st, 3rd and 4th byte lanes. but AHB has to do two access, one is single byte and 2nd is half word acess.
this is just one of tricks. good luck.