I think marquess's suggestion is probably the most likely especially as the style is clearly wrong, but I will offer another possibility:
If the groom's elder brother, Viscount Moore, has died (escaping the notice of this group), then Garrett would be heir apparent. Whilst he should be known as Viscount Moore in that instance, he might opt not to use that courtesy title out of respect (this has happened in other families), and instead uses another title possessed by his father a courtesy. However, they would both be known as Lord Moore in general conversation.
I think my suggestion is unlikely as I think we would have picked up on such a death, but I mention it anyway.