The rubrics for a low/high mass are 3 rings. The 1-3-1 is sometimes used at a solemn high mass, for added solemnity. Otherwise, it's probably an typical-french-sspx-influence where they use the 1-3-1 for low masses. The french influence in the sspx cannot be overstated and it shows itself both in liturgy and in theology. Sometimes good, sometimes bad.