John says it very well. There is the 'sag' element to the Roubaix unlike other bicycles. Sag is common to motorcycles when setting up suspension for fore/aft balance and ride height. Different riders will load the front more or less depending on rider weight and how aggressive the position is. That is why on a size 58cm Roubaix, stack is 644 and not 621. Loaded the effective bar height will be reduced on a FS Roubaix and why they made the front of the bike taller in its unloaded state. Net fit will be the same and there is a lot of adjustablility...including a lower stack height head set and of course you can run a -17 deg stem like many pros on pure race bikes to get your bars even lower. As John said, if you want even more saddle to bar drop, the Roubaix is not your target bike.
It could be argued for a pure race bike, the FS shock may not even be desired in fact.