They, I presume, intended to reference each separate datum; the first being "80% chance Bolsonaro wins" and the second being the most likely expected split.

I would guess they encountered it via a vaguely classical adjacent education or reading background, in that a (non geodesy / survey related) datum is a singular thing given, granted, assumed as fact, used in a reasoning chain.

More simply, the singular of data.

S'all Latin to me.