Here we have a unit of orange Horrors screening the unit of Pink Horrors sitting on the objective. This formation is to ensure that the Grey Knights can't run in there and contest it. Let's assume for this demonstration that the Grey Knights have the objective secured rule, but the horrors don't because of the way that their detachments are set up.

As we enter the charge phase, the Grey Knights rolled the 7 average on 2d6. I even busted out the handy dandy tape measure to show you exactly how far 7″ can get you, nowhere near close enough to get to within 3″ of the objective. However, you're a good player, and you've been keeping up with your Art of War classes in the War Room, so you understand that after killing the orange horrors, the Grey Knights will be able to pile in 3″ further to potentially get within 3″ of the objective!