Other than this (also works for levers), these types of pens usually didn't fill completely. The ink sack was also much smaller than our contemporary converters (and bigger systems like pistons). People just used to fill them more often and would not care that much to get it as full as physically possible. Ink wells used to be more abundant as well in many places. If their pen is vintage inspired, it may just be part of the charm. For example, those old Waterman pens have a small capacity but gush ink like there is no tomorrow. You'd better have a bottle of ink close by.