I define a “tick” of movement to be about the amount deducted to move one yellow square beyond blue range.
Case 1: Assault/Sniper with overwatch costs one less action. Move for 1 action point, fire for 2 Points, not enough action points to overwatch. (i.e. one “tick” less than a full action point.) I have also noticed it with different characters where I would expect to be able to fire a pistol but could not. NOTE: This NOT related to Case 2, NO enemy sighting interrupts.
Case 2: Character move is interrupted for enemy(ies) sighted. Loses a fraction of action point. First move of battle can lose most of an action point worth of distance after 4-5 interrupts. Most commonly experienced with stealth character on first turn of battle.