The difference between theory and practice.
In theory you may need one negative click at 25 yards but what does that equate to? If one click is a sixth of an inch (near enough) at 100 yards, it's a trwenty fourth part of an inch at 25 yards. That is about 1 millimetre.
In practice, you don't need to worry about it. If you shot a group of ten shots at say 0 clicks and another group of ten at -1 clicks, you might just perceive a shift of the group centre. The highest of the low group will be higher than the lowest of the high group. In other words, they overlap.
But when you shoot at a target, you have only one shot. It could have come from either group.