Well, lets see... the USMC is replacing 100% of its Hornets & Harriers with F-35.
The USN is only replacing "most" of its older Hornets with F-35... it is keeping all its Super Hornets, and the extra SHs it has bought means that F-35C will make up about 1/3 of the USN's fighter force (not counting the 80 Marine aircraft).
Oh, and the 80 USMC F-35C will actually be, for all practical purposes, USN aircraft... since the USMC is being required to operate them from USN CVNs to take the place of F-35C squadrons the USN isn't equipping!
So basically, the USMC gets only the 340 F-35Bs, while the USN gets its 260 F-35Cs AND the 80 USMC F-35Cs... but doesn't have to pay for purchase or operation of that last 80!