Is "volume" the quantity of BTC exchanged, or the number of trades completed?
They're both representing the same thing over different time periods. The values on the drop down are always for the past 24 hours, the values on the graph depend on what time range you're viewing.

In your example 75584 is the number of bitcoins traded in the past 24 hours, and 17261 is the number traded between 19:00 and 20:00