Is RTB Inventory volitile?

Here is a snapshot of inventory delivered across multiple RTB exchanges over a 45 day period. Notice that they all seem to follow the same directional trend however some are much more steady. Ad Meld and Pubmatic are the stable standouts while Ad Nexus wins for widest swings. Google is clearly winning for overall share of RTB.

*Since Right Media isn't a RTB service (yet)it was not included in this data set.