Odd Frame Rate 29.68 fps in Live Photos
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.