Because of where the judgement call is made. Here's an analogy -- in football, a first down is when the offense moves the ball at least 10 yards from the location at the start of the down. This is the baseline. Most of the time it's pretty clear if the ball is under that or over that -- situation where everyone would agree one way or the other. There's a judgement call when you're nearby, based on where an official spots the ball, that could result in being over or under the line. This largely encapsulates my position. @Oofta's position, as presented, is that it's always up the official when a first down occurs -- there is no baseline of 10 yards, it's just whenever the official determines that this is good enough.
So, sure, there are judgement calls in both, but it's frankly ridiculous to say that because there are judgement calls the approaches are the same.
Your analogy seems inapt to me. The judgement call on whether or not an invisible creature is hard enough to locate to warrant deviating from a default isn't inherently a question of numerical measurement. You might decide to create such a numerical threshold at your table (e.g. beyond x feet the default doesn't apply), but a DM using @Oofta's approach could create an equivalent threshold (e.g. beyond x feet an invisible creature's location is unknown).
So while I agree that DM determinations will vary wildly from DM to DM, I don't think the degree of variation at a particular table depends on whether the DM is describing their approach as determining whether to depart from a default, or determining the result directly. For any metric a DM might apply to determine whether or not to depart from a default, an equivalent metric exists to determine whether or not the invisible character's location is known.