A rolling total, not a table
Most sports rank teams by results within a defined season that resets to zero. Tennis does something different: each player carries a running total built from their best results over roughly the past year, and that total updates continuously as tournaments finish. There is no reset date and no shared starting line, which is why a ranking can move sharply for one player in a week that changes nothing for their nearest rivals. The number attached to a player at any moment is really the sum of several separate expiring contracts, each one a tournament result due to drop off exactly a year after it was earned.
Why points expire one event at a time
Because results fall out individually rather than all at once, a ranking can fall even when a player has done nothing wrong recently. If they reached a final a year ago and skip the equivalent event this time, those points simply vanish and nothing has arrived to replace them. This is the single most misread feature of the system: a drop in ranking is frequently a story about a calendar anniversary rather than a story about current form, and the two explanations look identical from outside.
Reading the list correctly
The practical consequence is that a ranking answers a specific question well — who has accumulated the most over a defined stretch — and answers a different, more tempting question badly: who is playing the best tennis right now. Treating the two as interchangeable is the most common mistake made about the sport's headline number. A player can be ranked below someone they have just beaten, and both facts can be true at once without contradiction, because they are measuring different things over different windows.