The volume counter is a real time display of volume over the last 24 hours. It might occasionally decrease as particularly large orders drop off as the 24 hour clock progresses.
For example, you look up and see the volume is 100,000. But exactly 23:59 ago someone did a 1,000 bitcoin trade. If you wait a minute or two and refresh, you'll see the volume drop to 99,000 assuming no one had any orders fulfilled in those two minutes.
This is a fairly simple explanation, but hope it helps! If you'd like more detail just let us know, or contact firstname.lastname@example.org