setoutpoint have one frame offset

I want to set in/out points for an image and add it to the timeline, but I've noticed occasional one-frame discrepancies. My specific steps are as follows:

setInpoint = 0

setoutpint to 14000 frame, using ticks, calc by timebase * frames.

insert to a 23.976 fps timeline, the image will only length with 13999.

The ticks should be the most accurate, but there still seems to be a one-frame deviation.

Inserting frames 10000, 13990, and 30000 works without errors. However, frames 14000 and 14899 do produce errors. It seems to be due to a rounding error somewhere.

Does an image have a frame rate? It seems not? Or is it a problem with how it's calculated?

I also tried obtaining the time object of the next segment and its ticks to fill in the gaps, but there was still a frame discrepancy, which is quite strange. 

Does anyone have any thoughts on this? Thanks.