I wonder if the capacity was based on how much each area holds per sqaure metre. Without taking in to consideration barriers, walkways etc.
Anyone at Guiseley would have felt another 3k in there would have been mental.
FYI The capacity of all grounds is calculated based on a number of factors as detailed in the green guide, these rules have not changed in ages. These include terrace areas, terrace dimensions, terrace condition, barrier design, loadings and condition, P and S factors and the capacity of entry turnstiles, exit gates, emergency exits rate of exit and spectator flow rates. If pitch barriers are not load tested then theoretically they are not safe; in reality where grounds have minimal standing and low spectator numbers the risks are extremely low of pitch fences collapsing, but it can and has happened. Some of our pitch fences are in front of emergency exit routes, such as golf rd end, away end and other parts where anyone standing at the fence will obstruct the exit route.
Hope this helps.