Or, a deer jumped out on the side and you briefly looked away at it.
Or you could tell the driver behind you wasn't slowing down, so the safer option is to go.
Or. Or. Or. Real life is messy, and there's a million reasons to go though a yellow instead of slowing down.
This is common in the US as well. The machine takes the picture, filters out the illegible ones, and sends the rest to an actual officer who will issue the ticket.