“In traditional engraving, when a bar consists of a single note and there are no other durations in any part, the note is placed just to the left of the center of the bar. Such bars, by definition are usually fairly narrow. This spacing creates a better balance than a single note positioned at the beginning of the bar:

In widely spaced bars, the note can be placed closer to the barline so as not to appear isolated. When there are other durations in other parts, the single duration is placed at the beginning of the bar as normal.”
from : Behind Bars: The Definitive Guide to Music Notation
by Elaine Gould (pg 41)
more >> “Archaic Techniques : Note Spacing : Centering Notes of Bar-Length Duration”
