A liquid in a U-tube maintains the same level in both the arms. When another liquid which does not mix with the first one is poured in it, the air interface with either of the liquids in both the arms will be at different levels. Considering a point at the top surface of liquid A, the pressure there should be atmospheric pressure. Another point in liquid B taken at the same level will have a pressure that amounts to atmospheric pressure plus the pressure due to the column of liquid B over it. These cannot be same. So how is the pressure at the same depth not being equal when we use that principle to solve many problems.