Other mistake: When the two Americans are walking on the moors, their shadows are always in front of them, despite them having changed direction numerous times. Additionally, with the moon always being behind them, how are their faces being lit?
Other mistake: When David changes into a werewolf for the first time you can see his huge feet vibrate when he moves his legs in agony: they are made from rubber.
Answer: Traumatic events have a tendency to disrupt the memory - people who have been involved in a car accident, for example, often have no memory of the events leading up to the accident, even though they would obviously have perceived those events at the time. The physical and mental stresses of the werewolf transformation have clearly disrupted David's memories in a similar fashion.
Tailkinker ★