What genius came up with that statistic?

Last night on Nightline, in a piece about diamonds, the narrator said that most engagements and proposals happen this time of year. He backed this up by saying "December and the run-up through Valentines Day accounts for 25% of all engagements." So, let's see, Valentines Day is in February, and January comes between February and December... So three months account for 25% of all engagements that occur in a calendar year. Hmmm...