Let's take two extreme cases:

Case 1: column supporting 20 stories

Case 2: column supporting 1 story.

It's clear that the probability that column 2 is fully loaded (LL) is greater than the probability of column 1 is fully loaded.

That's why columns At must take into account the whole area supported by the column, and not only the tributary area of one story. Since At is in the denominator, when At increases the reduction also increases which is in coherence with the principle behind LL reduction :

"The probability of full live load (except storage, parking and assembly area) is unlikely when influence area is larger" than a particular area specified by the code.