Here's what happens. When the Drobo is connected to a computer and the Drobo Dashboard software isn't running, the Drobo's capacity meter will overestimate the used space, potentially triggering a low space alert. When the Drobo Dashboard software is started, it does its own used space calculations and corrects the capacity meter, literally turning off one to two or even three of the blue LEDs that indicate how much space is used.

More info: raoulpop.com/2008/06/06/drobo-overestimates-used-space/