It does appear that the timing specifications for cross-country and alpine are slightly different.
My understanding is that photo finish systems (line-scan camera system) are currently only used as backup systems for FIS alpine events. These would note when any part of the skier crossed the finish line.
But today photocells are specified as the primary system, with both targeted below the knee.
From the FIS timing booklet (alpine)
The cells must be mounted so that both beams are triggered at a height
that is lower than the knee of racers at the finish. It is recommended
that the lowermost photocell be connected to Timing System A.
This information could be out of date for the 2022 Olympics, but the 2022 US Ski & Snowboard Advanced Timing Study Guide suggests that it hasn't changed yet.
Photo Finish timing has been approved for use at FIS events for backup
It's hard for me to tell specifically what the purpose of the skiers position is at the finish, but I wonder if they are leaning down and trying to trip the cell with their hand rather than with their leg. On some finishes I watched, the arm was down very low compared with someone just trying to lean forward and trip with helmet or chest.
Also, I believe cross-country events are currently timed from the boot, not the ski tip.
Nordic Combined (Cross-Country)
The following electronic timing technologies can be used to identify the official finish
- Electronic timing system based on photocells. The measuring point of the light or
photo barrier must be at a height of 25 cm above the snow surface.
- Photo finish system. The measuring point will be the toe of the first boot meeting
the finish line