[FFmpeg-user] Why are PTS values different from what's expected?

[Mark Filipak (ffmpeg)]

The source is MKV. MKV has a 1/1000 TB, so any PTS variance should be less than 0.1%.

The filter complex is thinned down to just this: settb=1/720000,showinfo

Here is selected lines from the showinfo report (with   ...comments):

[Parsed_showinfo_1 @ 00000247d719ef00] config in time_base: 1/720000, frame_rate: 24000/1001
    ...So, deltaPTS (calculated: 1/TB/FR) should be 30030.
[Parsed_showinfo_1 @ 00000247d719ef00] n:   1 pts:  30240   ...should be 30030
[Parsed_showinfo_1 @ 00000247d719ef00] n:   2 pts:  59760   ...should be 60060
[Parsed_showinfo_1 @ 00000247d719ef00] n:   3 pts:  90000   ...should be 90090
[Parsed_showinfo_1 @ 00000247d719ef00] n:   4 pts: 120240   ...should be 120120

The PTS variance is 0.7%.

Why are PTS values different from what's expected?

Note: If I force deltaPTS via setpts=N*30030, then of course I get what's expected.

Thanks. This is critical and your explanation is greatly appreciated!
Mark.