That's because Sport mode is for panning it when riding in a truck and shooting such as if in a truck on a safari. Sport is 2 axis (side by side) whereas Normal will help stabilize the up and down movement as well.
Can you find anything from Nikon that confirms your understanding that Sport is
only for panning?
You may be confusing the ability of Nikon in lens VR to detect panning and not to apply VR in the direction of a moderate speed pan.
Most Nikon VR in lens optics going back almost 20 years have had "panning direction VR disabled" ability - whereas Sport is a relatively recent mode.
Instructions for relatively recent lenses such as the 500 PF say panning motion is detected in either Normal or Sport.
For the 105 S the instructions unhelpfully say refer to the camera manual. The instructions for the 70-200 S mention Normal and Sport but do not clarify the difference.
The Z9 extended manual does not seem to clarify the difference.
The Z7 manual (first version page 112) clarifies "panning direction disabled" applies to both Sports and Normal modes.
The VR stops advantage for recent lenses like the 70-200 S in the Nikon Specification is mentioned as being for Normal mode.
This strongly indicates to me slightly less VR ability in Sport mode
If you use the normal mode, you will see the first image taken always jumps and in the sport mode there is no movement in the viewfinder. This is the same either for DSLR and Mirrorless.
This is news to me - as a user of Nikon VR lenses for around 20 years :-O
My experience is VR can occasionally cause a viewfinder "jump" - most likely after taking a shot with VR on.
I just tried the my Z7, Z9, 500 PF (VR control on lens) and 105.70-200S (VR control in the camera menus) - and hand holding I cannot detect a "jump" difference between VR/IBIS off, Normal and Sport.