This man found a surprise in his egg
Do you take your eggs boiled or scrambled? If your answer was boiled, something like this could have happened to you. While peeling their egg, this person found a second tiny egg inside. If they had cracked it to scramble it, some shell pieces might have been left in the pan.
Although the egg on the outside was the same size as a regular eg, it was able to hold a second egg inside. Whoever bought it paid for only one egg and got one for free. To an extent, despite there being no real gain (besides a cool anecdote), freebies are always satisfactory.