Millisecond duration calculation discrepancy

I have a video that is 59.94 fps. 

As part of my workflow, I add a graphic to the timeline that will later be used to indicate to a post-production team to insert some snazzy CGI. I'm trying to create something that will calculate how long that graphic is present so that I can keep logs of how long these sections are (in milliseconds)

An examle of the issue:
Graphic Start: 00;02;44;06 (from the original timecode, HH;MM;SS;ff)

Graphic End: 00;02;46;45 (from the original timecode, HH;MM;SS;ff)

So length is 2 seconds and 40 frames.

Premiere Pro calculates this to be 2669.33 milliseconds

My calculation, using frames/framerate + time is (40frames / 59.94fps) + 2000milliseconds = 2667.33 milliseconds

Is anyone able to account for the missing 2 milliseconds?