AP- Why exchange rate type between Post invoice and Post payment is differe

AP- Why exchange rate type between Post invoice and Post payment is different ?

Hello,

I have question

when post AP invoice , document type KR

document date = 11.12.2009

posting date = 31.12.2009

entry date = 05.01.2010

this document using exchange rate type B (standard translation at bank selling rate) on 31.12.2009 (use posting date )

but when post payment for above document,document type ZP

document date = 25.06.2010

posting date = 25.06.2010

entry date = 24.06.2010

this document using exchange rate type M (standard translation at average rate) on 23.06.2010

please answer

why do post invoice and post payment use different exchange rate type?

and why at payment donot use posting date for get exchange rate ?