Thanks for all the replies - I had no idea I was opening up such a can of worms! I can see the point some are making about people who are really in the bottom two, but not named as such, being at an unfair disadvantage. They may have gone out anyway, we don't know, but they don't benefit from any possible "bounceback" sympathy vote the next day which may or may not make a difference.
The only possible reason I can see is if they are so far behind the others in the vote that they have no way of catching up even with a normal degree of "bounceback" that the producers want to pull the plug on them, and choose to give a possible advantage to someone else for whatever reason.