Reaching a World Cup is not a single global competition with one standard of entry. Each continent runs its own qualifying tournament for a fixed number of places.
Places are allocated before anyone plays
The world governing body assigns a quota of berths to each confederation, and those quotas are set well in advance of qualifying beginning.
Allocation reflects a mix of competitive strength, the number of member associations and the political weight of each region, rather than a pure ranking of quality.
Because the quotas differ, a team of identical strength can face a comfortable path in one continent and a brutal one in another.
The formats differ as much as the numbers
Some confederations run a single long league in which every contender plays everyone else home and away over a period of years.
Others use multiple rounds of groups, progressively eliminating smaller associations before the established teams enter, which shortens the schedule for the strongest sides.
A few regions decide places through a compressed final round among a handful of teams, where one poor week can undo two years of work.
North American qualifying has its own character
The regional confederation covering the United States spans North America, Central America and the Caribbean, and includes associations of very different size.
The competitive difficulty is less about the strength of opponents than about conditions, since away trips involve heat, altitude, small stadiums and hostile crowds.
American teams have long treated the away fixtures as the real test, because home results against the same opposition are rarely in doubt.
Playoffs connect the continents at the end
Several confederations do not receive whole numbers of places. A fractional allocation is settled through intercontinental playoffs between teams from different regions.
Those matches are often the only competitive meeting between the two continents in the cycle, which makes them extremely hard to forecast.
They also tend to be scheduled close to the tournament and at neutral or single venues, removing the home-and-away structure that qualifying otherwise relies on.
Why expansion changed the calculation
Enlarging the field increases every confederation's quota, but not proportionally, so the relative difficulty of each path shifts.
Regions that previously sent only their strongest teams now send several, which raises the number of debutants and widens the range of quality within the tournament itself.
It also changes what qualifying measures. A path that once identified a continent's single best team now identifies its leading group, and finishing fourth or fifth in a region has become a meaningful competitive goal.