Why pension and not CPF scheme for ministers?

Yahoo! Singapore News, 22 Nov 2011
Prime Minister Lee Hsien Loong said that retiring ministers must serve at least eight years in office to be eligible for a pension.
Replying to a question by opposition Member of Parliament for Aljunied GRC Pritam Singh on Monday, he disclosed that the pension is based only on the pensionable component of the salary and not on the total monthly salary.
This pensionable component has been fixed since 1994, and so it has been decreasing in proportion to the total monthly salary,... Full story