I thought it was more that when on sea, they have three moves and when they land, they still have their two moves. While, if they had two moves at sea they use one to land and have one left. They get free moves while at sea and landing, but they can't get the sea moves on land.
Unless I'm mistaken and that's actually the case. I haven't tested it.
The bug is that the Danes maintain the remaining sea moves after disembark. So, if you have 5 sea moves and disembark, you have 4 moves which is way overpowered. You can see this in the image I posted above in a response to this thread. The movement points for my infantry are 4/2, which is clearly a bug (they should be 2/2 at maximum).