The only thing I could think of was that they wouldn't end up using the eighth gear at some circuits (Monaco, Hungary for example). It definitely sounds ridiculous, though.
Reading into it a bit more, it seems this is the case. The ratios will be fixed, but to allow this to cover all tracks the number of ratios is changed to 8. So yes, there will be tracks where 8th gear is not used.
I can see the thinking behind this. Teams wills start the season with a number of engines and gearboxes in crates that are all identical, and mechanically cannot be changed. This reduced costs and ensures parity between customer and factory teams gearboxes. But it just feels a bit odd.
Could we even see teams perhaps sacrificing races at one end or the other (Monaco/Monza) in order to gain small advantages for the other 19 or so races?