Thats actually a rather common problem.
Files typically have two ARs encoded, one is the container AR, and the other one is the stream AR. You can "usually" trust the container AR, but the stream AR is most of the time just wrong (especially in H264). The exception to this rule are MPEG-TS files, where its reversed.
Now the problem is that most decoders do not offer you can option to ignore the Stream AR and only use the Container AR as provided by the source filter. The Haali splitter uses some rather ugly hack to work around this - it actually modifies the H264 bitstream and overwrites the Stream AR inside with the Container AR, so the decoder has no choice but to use it (at least if its configured to do so, but its default on, iirc). I'm usually no fan of such hacks, so i didn't implement this kind of hackery in LAV Splitter. The perfect solution is of course if the decoder offers you an choice between Stream AR and Container AR.