I'll try to answer my own question. I'm not sure this is 100% right.
I suppose frame rate's issue depends on the architecture of Live Photos itself. Total number of frames inside each Live Photo in iPhoneX is 85 frames (look at the picture above):
42frames + 1frame + 42frames = 85frames
Frame rate for all these 85 frames is supposedly 30 fps, but considering just a video part (without a HEIC file), frame rate is 29.68 fps.
Here's a math:
//let's assume that 1/85th is 0.011764,
//truncate that floating point number to 0.011
1 / 85 = 0.011
//let's assume that video frame rate of all 85 frames is 30 fps
//so each frame's contribution is approximately 0.33 fps
30 fps * 0.011 = 0.33 fps
//thus H.265 video portion without HEIC frame will be 29.67 fps
30 fps - 0.33 fps = 29.67 fps
Value 29.67 fps is very close to 29.68 fps.
I reckon that altogether all 85 frames of Live Photos have frame rate 30 fps, but two H.265 video portions (without HEIC part) have frame rate 29.68 fps.