A) EK is not a charity, but a business. They OpUp ,( like every other airline ), when they need to make room and sell some seats
B) The reason they send those "Upgrade for only $XXX" emails, right before departure, is to entice customers to pay for an upgrade on over booked flights
C) In order to increase your chances, make sure you book on packed flights [ Just switched from EK373 to EK419 because its packed ( according to seatcounter.com )
D) OpUps in reality is the revenue management department doing its job, not a gift from "the business" to "the customer"

Although in all fairness, OpUps with EK are a hit & miss. Been sure that would be opped up at the gate only to discover that there were a billion golds on that plane ( smile and the crew will let you take a peek at the manifest )