Consider the interval D3 G3. This would form a perfect fourth.
In a sequential interval, say a quarter note D3 followed by quarter note G3, this still "sounds" like a perfect fourth so using that interval label seems fine.
However, suppose instead the sequential interval is D3 followed by G2, thus the second note is lower pitch rather than higher. Would it be more appropriate to label this interval as perfect fifth instead of a perfect fourth?
In other words, should an interval be labelled based on the lowest pitch, regardless of whether it comes before or after another note in time?