The depth of your map card is not always going to exactly
match the depth of the water you are in.
The water levels of all bodies of water will
fluctuate depending on rain fall.
In times of drought, water is drawn from some reservoirs for
drinking purposes.
Also, when the map cards are created, they are probably created using
some type of average water height. Maybe average for 1 year or average for
5 years. Who knows?
There are also map card errors, and GPS positioning errors to take into account.