Hi
There are two reasons for this.
First, as LDFerguson said, it may be the market value adjustment which applies to certain guaranteed funds. It doesn't apply on retirement, only transfers.
Second, on older plans, rather than high upfront charge the charges of the pension are spread over the term of the pension as dictated in the T&Cs (usually to age 70, even though most people retire earlier.) The T&Cs read like something stephen hawking put together. You will be paying these charges gradually to retirement, or all at once if you transfer out early.