Why is peripheral clock frequency is different than oscillator frequency in PIC microcontrollers?
It's because The CPU needs 2 (dsPIC33/PIC24F) or 4 (8 bit PIC) clocks per instruction cycle, but the peripherals only need 1 clock.

On 8 bit PICs each instruction cycle consists of 4 'Q' cycles:- 


Q1: Instruction Decode Cycle or forced No Operation 
Q2: Instruction
Read Cycle or No Operation 
Q3: Process the Data 
Q4: Instruction Write
Cycle or No Operation


Each 'Q' cycle requires 1 oscillator clock, so instruction cycle frequency (Fcy) = Fosc/4. 

This is not related to pipelining, which is applied to the entire instruction cycle. 8 bit PICs have a simple 2 stage pipeline where the next instruction is fetched at the same time as the current instruction is being executed.



dsPIC33/PIC24F have a multistage pipeline which starts fetching the instruction half way through the CPU cycle, then takes up to 9 cycles to complete the fetch and execute. On these MCUs Fcy = Fosc/2.

    

Compared to other MCUs which have equal instruction and clock cycle timing, 16 bit PICs are 2 times slower and 8 bit PICs are 4 times slower. An 8 bit PIC running at 64MHz is equivalent to an 8 bit AVR running at 16MHz.