Each fraction is a mix of hydrocarbons and ach fraction has its own range of boiling points and comes off at a different level in the tower.
Notice that the ranges overlap. This is because the distillation process is not exact - some C-30 hydrocarbons will condense at the level for light gas oil and some will condense lower down.
This overlap isn't very important for the heavier fractions. However, the lighter fractions, like LPG, are usually separated more tightly in another fractionating tower.