In some AVI files, there's a chunk named 'vedt' placed at the end of the header. You can download one of these files here: https://samples.mplayerhq.hu/V-codecs/RLE/filecopy.avi . In this example, the contents of the chunk in question are just zero. There are also other files of the same nature within the sample directory. However, https://samples.mplayerhq.hu/V-codecs/RLE/filedelr.avi contains some non-zero values within the 'vedt' chunk.
Does this have something to do with masking the background so it can played with a user-defined background? that's what I suspect.