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Hi,
the dive computer calculates the depth based on the barometric pressure to ensure a correct measurement at altitude and for weather related variance. If the barometric pressure increases during the dive the depth reading may be a bit off when you get back to the surface as it doesn't exactly match the starting value. As long as it shows 0m when you start your dive the measurement will be correct. If it's off after the dive make sure that the water contact is deactivated and the device is in time mode so that it can readjust.
br, Hendrik