ffmpeg timing metadata values differ from the ffprobe output

When printing the ffmetadata metadata of an audio file with the ffmpeg command:

ffmpeg -i INPUT.mp4 -f ffmetadata FFMETADATAFILE

the approximation of the timing values fields start and end in the [CHAPTER] object:

[CHAPTER]
TIMEBASE=1/1000
START=533975
END=586975
title=Meltdown/Spectre patched

is different from the same timing metadata, obtained using the JSON output with ffprobe the command:

ffprobe -i INPUT.mp4 -show_chapters -print_format json

that is

  {
        "id": 2,
        "time_base": "1/1000",
        "start": 534000,
        "start_time": "534.000000",
        "end": 587000,
        "end_time": "587.000000",
        "tags": {
            "title": "Meltdown/Spectre patched"
        }
    }

where start_time is start and end_time is end of the ffmpeg output. Is there a way to handle the timing info the ffprobe metadata output?