FIDIC: What is the difference between "the amount certified by the Engineer" and "the amount which is actually due"

FIDIC Contracts
1

Under CONS/P&DB, reference is made to “the amount certified by the Engineer” while under EPCT, the reference is made on “the amount which is actually due”.

I would like to find out the difference between these definitions of “amount” as both are certified by the Engineer and the Employer’s Representative respectively in the Interim Payment Certificates based on the Interim Payment Applications submitted by the Contractor.

2

In the Red and Yellow books (CONS and P&DB), the Engineer certifies the amount payable, i.e. it is an amount decided by the Engineer. In the Silver book (EPCT) there is no Engineer and so the amount is decided by the Employer.

The difference in drafting reflects those different positions.