Stock indices fall on lower volume

Stock indices fall on lower volume
Above is a title in a article. I would like to know why “lower” is used here?
“Fall” mean that it is lower and therefore it seem to be redundant, right?
And “lower” is a comparative form of “low” but I don’t see the object compared.
Can I rewrite it like this?
Stock indices fall on volume

“lower volume” means that fewer shares were traded on that day (or whatever period is being referred to) compared to previous days or normal days. The fall of the indices is a different measurement, based on share prices. The slightly vague or journalistic use of “on” associates the fall of the indices with the lower volume, without actually stating cause and effect.

Your rewrite is a bit too highly abbreviated for comfort, I think.

Thanks Dozy for the detail reply! -:slight_smile: