The first value is the unmodified CON, the second usually applies to some form of armour (or tough/scaly skin on a Lurker) - if the weapon used has an AP value (such as the AP of 1 for the knife or 2 for the axe) & if its AP is equal/greater than the CON difference given by the higher value over the lower, (for the Formless Thing the increase is 2 points of CON) the target's CON drops to the lower value - so an axe, with an AP of two would drop the Formless Thing's CON back to its 'natural' 6.
As to where it is in the rulebook, I'm afraid that without it to hand I couldn't say.