We are measuring 50 ml of water or 50 ml of milk or 50 ml of oil with the same measuring jar. So the quantity in volume is same. Ideally when these liquids are poured into the same glass separately, all the liquids will rise up to the same glass mark/level. Ideally the volume of oil in the glass is also 50 ml.
Due to higher viscosity, oil does not completely get poured into the glass. some oil remains in the measuring jar as it sticks to the walls and the bottom of the jar. But Water has the least viscosity of the three liquids, so most of it gets into the glass.