Virtualdub problem

I am fairly used to using Virtualdub - if I have a movie that comes with
VBR audio - I rip the audio to a wav file then remux with the video using
Virtualdub - I compress the audio wav file using Lame 3.961 codec and that
usually syncs and plays in my standalone DVD. I am getting a problem (I
think !) with a few films recently - I choose 160 CBR in Virtualdub (Lame)
when I encode, but if I inspect the resulting file with AVICodec, it
sometimes seems to show as a VBR audio ?

Not sure what is going on - can anyone help please ?

Thanks for any help
.