Adding to Glenn’s answer…
The assessment of value of a CE is not necessarily directly linked to the assessment of time. They actually serve different purposes and are calculated differently.
The change to the Completion Date is primarily a defence against delay damages (or general damages if there are no delay damages). The changes to the Prices are compensatory as against actual Defined Cost incurred and a forecast of Defined Cost to be incurred. So, if the impact of a CE is to move planned Completion 10 weeks that does not necessarily equate to the same period or full prelims being used as the forecast. The prelims could be forecast at a higher rate for a shorter period so that the contractor creates terminal float. Equally the prelims could be the same or lower over the same or shorter duration (or even a longer duration).
The point Glenn makes which is absolutely correct is that once the CE is agreed that is it, you do not go back and adjust for what actually happened.