Hello
When exporting the depth data to CSV file, the depth shows up in two different values with 1m difference in the same location. Why is this happening and which depth is the correct one ?

That's an isobath, which is a depth area between two depth values, so the two values you see are the lower and upper value for that range. If you want individual contours with a single depth, then you should export a contours layer. Major contours and isobaths share the same paths if they are generated together as part of the same map.