With lower descent speeds the rate of descent is lower, therefore the time spent at lower fuel burn is longer (and less time spent at cruise fuel flow settings). The FMC thinks it is more economical to do it that way.
But Airbrake has hit the nail on the head, if we descend based on CI 10, we start down early but are then asked to speed up which is generally in the lower levels. Here the fuel burn goes through the roof, nullifying any gain we would have made.
edit Sadly Cynical:- Also not quite correct as more fuel burnt at top of descent due to descending later with cruise thrust fuel flow for longer (see above).