A rivalry list that jumps from one decade to the next is not incomplete. The two clubs simply were not in the same division, and there was no mechanism to make them play.
Fixtures require sharing a competition
Clubs meet in the league only if they are in the same tier that season, and one relegation is enough to end a fixture that had been played twice a year for a generation.
American leagues have no equivalent, since a team stays in its division regardless of results and the schedule guarantees certain opponents every year.
The consequence is that a European head-to-head record has holes in it, and those holes are informative about the clubs even though they contain no matches.
The gaps are not randomly distributed
A club spends time outside a division because it was struggling, so the missing years are usually its weakest period rather than an arbitrary interruption.
That biases the surviving record. The meetings that exist are the ones where both clubs were healthy enough to be in the same league at the same time.
Reading a balanced record from those matches therefore understates how far apart the two clubs have been for much of their shared history.
Cup draws produce the isolated meetings
During a gap, the only way the clubs can meet is through a domestic cup, where the draw is open across divisions and pairs any two entrants.
Those matches are memorable precisely because they are rare, and they are heavily overweighted in supporters' memory of a rivalry.
They are also structurally unusual, since one club is typically fielding a rotated side while the other treats the tie as the biggest match of its season.
Returning fixtures are not resumptions
When a promoted club meets an old rival again after several years, the fixture is often described as a renewal, which implies continuity that does not exist.
Both squads, both coaching staffs and often both stadiums have changed, and the crowd contains a generation of supporters who never saw the previous meeting.
Treating the result as evidence about a long rivalry mistakes a shared name for a shared situation, which is the recurring error in this kind of preview.
How to read a record with holes
Count the seasons in which the clubs could have met but did not, and compare that to the number of meetings, because the ratio tells you how equal the two clubs have really been.
A rivalry with no gaps is between two clubs of similar standing, and only that kind of record supports the conclusions people usually draw from head-to-head numbers.