I think your math is wrong. The denominators vary for the different percents. (Edit: nope, mine was.)
Softbank buys 20% of their Japan space, so in order to get to 40+% they just need to find institutional investors for another 20+%? The denominator for both is "capacity at Japan office"
It looks like they just need 25% of remaining office space to be institutional to hit that (20 of 80).
That doesn't really affect your original point though.
If Softbank is renting 20% of space, and less than 100% of space is rented, then they count for more than 20% of the customer base being institutional.