That's true, it won't be a
huge 20' dramatic difference...but if you look at the plan...level 7 and 8 will be quite a bit different from the 6 below. I'm guessing this is why level 7 is taking so long...they are transitioning from the garage floors which were 9' floor to floor (8.33' floor to ceiling) and spiraled up to having to make it an even flat surface on level 8. Some areas of level 7 look like they will be up to 15.5' tall from the drawings...and from the street view it looks like its going to go from 8.33' to 12.08'...still a pretty big jump.
Ok enough rambling...in short, floor 7 + floor 8 = 27.67' combined...vs 9' for levels 2-6, so two floors will equal three previous ones.