Because the Rear Jacks are plumbed together, Right together and Left together, so as you lift the front the increased pressure on the rear jacks causes the fluid to flow from the them into those middle jacks, thus keeping the pressure on the frame equal at all times.
2018 Chevy 3500HD High Country Crew Cab DRW, D/A, 2016 Redwood 39MB, Dual AC, Fireplace, Sleep #Bed, Auto Sat Dish, Stack Washer/Dryer, Auto Level Sys, Disk Brakes, Onan Gen, 17.5" "H" tires, MORryde Pin & IS, Comfort Ride, Dual Awnings, Full Body Paint