The North Stand capacity seems to be restricted by the site dimensions. There is only an 11m walkway outside it to the site boundary which is at the narrow end by modern standards which demand vehicle entry & broad fan concourses for safety.To increase capacity you need to increase stand depth, losing paving.
As there is only a 20m gap at the South end and this has to house the car park access road effectively taking half the space (cannot tunnel in a dock) you cannot do much about it.
So the site effectively only allows one deep end in North/South configuration and that layout is needed for a riverside car park and then roadside fan plaza designed to ease rapid entry/exit for fans. Apparently an East-West layout, that better aligns to the site shape, does not easily create car access or a wide fan plaza for pedestrian circulation. These are both key practical design needs.
Explained to me by an architect mate at HOK who works on stadia/arenas. Every design decision creates trade-offs if the site dimensions are a limiting factor as they are at BMD (bigger than Goodison site, smaller than e.g. Emirates space). He thinks what he saw in that document was probably close to optimal space use for the site.
He also thought the elevated 'floating' roof design that seemed to be levered off the mid tier in the side drawings suggests that one or more tiers may be retractable underneath the roof of West & South stands if needed but says cost for that would blow the budget up nearer £500m (thinks it will be £400m not £300m anyway!).