If you have two discontiguous selections and one or both of them contains a single cell, hitting the enter key will not change the current cell from the singe selected cell(s).
To reproduce:
1) Select cell A1. Hold down the command key and select cell A3.
2) Hitting the enter key should switch between the two selections, but it will not.
If either of the selections contains more than one cell, the enter key will move through that selection to the last cell, then to the single cell. It will then not move out of the single cell.