For a very rough analogy, it's like standing right at the back of the noisy New York Stock Exchange and trying to listen to your buddy whisper a stock tip on the other side of the room.
Of course, you can't hear him. But, if you took the recording of the previous day's business hours, and subtracted that from the noise you're currently hearing, the noise would slightly dull. This would give you an opportunity to hear your friend's whisper.
(In real life, the stock exchange "noise" isn't "random," so technically this wouldn't work. But for now, assume it's random noise, and by the looks of our economy, that might actually be closer to the truth.)
However, you'd need to take a measure of several days' worth of exchange noise to really hone in on the whisper to avoid any inconsistencies in the "random noise" interfering with your hearing the stock tip correctly.
So, going back to Mercury's UFO, it's actually a false "stock tip" - more days of random noise subtraction are needed before the planet's ghost is removed from the image. Mercury is basically acting like the loud guy standing right next to you who keeps screaming the same stock tip over and over again; he introduces a huge uncertainty in what your friend is whispering.