Differences with Other Canons
ISTM that up until the Second Vatican Council, the numbering of the Roman Catholic editions of the psalter was that of the Septuagint. I'm fairly sure the Vulgate and preconciliar prayerbooks list the Miserere as Psalm L. The shift in numbering is a reflection of the increased participation of Roman Catholics in critical textual scholarship, which was until the middle of the twentieth century dominated by Protestants. (I personally lament this and similar well-intentioned but confusing and misguided shifts in numbering the psalter by Orthodox Christians.)