Fred and George stole thevtime turner.
Theory about GoF. Fred and George are really specific on the outcome of the world cup to the point they bet everything they have, even a toy wand. How are they so confident of the outcome. This was not luck. They knew the results ahead of time. With the marauders map they saw hermione in multiple places and followed her. They caught her using the time turner and eventually snatched it. They travel 2 yrs into the future, figure out the results of a major game, the world cup, return a second after they left, replace the time turner and leave. They save every knut they can for the bet, fred and george do not seem the type to save for a rainy day. This was their chance at financial freedom and they were super secretive of it, which they have been proven to be multiple times.
TLDR: the twins stole the time turner and cheated to win the world cup bet.