VdubMod - AC3 track length reported differently to 2 other programs

[Skywalkerjen]

VirtualDubMod 1.5.10.2 (build 2540/Release)
AviMux GUI 1.17.3
HeadAC3che v0.23a

AC3 is muxed to the video using AviMux in AVI container.

When the muxed avi is opened in VirtualDubMod, the streams list states that the AC3 length is 5.38.752

When the AC3 file is opened in HeadAC3che the length is reported as 5.39.136

If I encode the AC3 as a wav using PX3 Convert the wav length is reported as 5.39.104 in Sound Forge

If I demux the AC3 back out of the avi using VirtualDubMod the length reported by HeadAC3che is 5.38.720 - the same length as the video.

I need to know the accurate length as I'm joining avi's together so offset is required for audio tracks in the second avi. There is nearly a half second in some comparisons.

1. Which program is reporting the correct length?
2. Is the AC3 stream being reduced during muxing so that I'm actually measuring two different streams - the one muxed to avi is shorter than the original stream?
3. Which one should I use for calculating the offset?

Thanks for any help

EDIT: Is there a program that independently measures the length of DTS streams?