What causes the existence of two liquid levels in the glass plate level at the bottom of the hydrorefined diesel distillation column?

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The fractionation tower of the gasoline and diesel hydrocracking unit in my workshop is primarily used for product separation; naphtha is obtained from the top of the tower as a feedstock for continuous reforming, while refined low-sulfur diesel is produced at the bottom of the tower for use in fuel blending. The operating conditions at the top of the tower are 0.12 Mpa/135°C, while those at the bottom are 0.15 Mpa/275°C. There are 5 glass panels in this tower, from top to bottom; the liquid level can be seen on the fourth glass panel from the bottom, and it is also present on the fifth glass panel. The DCS indicates that the liquid level is low, with a difference of about 30% compared to the level shown on the fourth glass panel. The level gauge used is not a double-flange type. By adjusting the amount of refined diesel sent out from the bottom of the tower, it was observed that the DCS reading changed significantly, and the liquid level on the glass panels also changed (although to a lesser extent than indicated by the DCS). I would like to ask: why is there a dual liquid level reading on the glass panels? How should the actual liquid level in a distillation tower be determined?