The frames that are shown on the Sketchfab viewer don’t really reflect the actual framerate of the model, it just mimicks a “frame by frame” assuming that the framerate is 30 FPS (which is the case for FBX since we bake all FBX animation at 30 FPS)
For Blender, this info is not relevant since the software may be set to use 24 FPS playback.
Animation is converted from Blender “frame-by-frame” to Sketchfab “interpolated curves”, so a Blender frame becomes a time on Sketchfab, and this time should normally correspond to the value in Blender, as the processing reuses the
frame_fps setting in Blender.
If you want a 1:1 matching for frames between Blender and Sketchfab, you could set animation playback to 30 fps in Blender, but it will speed up the animation.
I hope it’s a bit more clear now