The gap is closing...

...amongst Korean teams.

In season 3 worlds there was one Korean team that stood above the rest (SKT)

In season 4 worlds there were two Korean teams that stood above the rest (SSW SSB)

In season 5 worlds all three Korean teams sent to worlds stood above the rest (SKT KOO KT)

In season 6 worlds all three Korean teams sent to worlds plus one that wasn't sent (4 total) stood above the rest (SKT ROX KT SSG)

Every year Korea has one more strong Korean team so the gap must be closing over there.