Teddy I checked with Zak of Mares. (Unfortunately in this aplication), the algorithm that Mares uses assumes that you aclimatise at altitude before diving. This means you have less nitrogen when entering the water than you would if entering at sea level.
To use the compter at sea level but set in altitude mode ignores this extra nitrogen loading. I never did the sums but remember reading that this can be equivalent to a couple of pressure groups.
If Mares set it up to account for sea level loading it would be ultra conservative for those lodging at altitude.