Unexpected behaviour when using expression in duration-attribute

I have setup a test scenario where incoming events update a fact that maintains an average value. If this average value is greater than a certain threshold for 1 second, a rule triggers (using the duration-attribute). The inserted events are configured to validate the rules condition immediately. When using a literal duration time value, the rule triggers correctly after 1 second. When an expression is used that resolves to a global that contains the same value, the rule triggers only after 20 seconds. (note that 20 events are being inserted). So it seems that an additional delay of 1 second is added to the duration length for each new event. Code samples to reproduce this issue can be found here: https://dl.dropbox.com/u/1899207/droolsdurationtest.zip Output: [1346921201633] : Created event stats intance [1346921201638] : Processed new event [1346921201830] : Processed new event [1346921202030] : Processed new event [1346921202229] : Processed new event [1346921202430] : Processed new event [1346921202630] : Processed new event [1346921202640] : duration rule with constant triggered after 1019ms [1346921202830] : Processed new event [1346921203030] : Processed new event [1346921203230] : Processed new event [1346921203430] : Processed new event [1346921203630] : Processed new event [1346921203830] : Processed new event [1346921204030] : Processed new event [1346921204230] : Processed new event [1346921204431] : Processed new event [1346921204630] : Processed new event [1346921204831] : Processed new event [1346921205031] : Processed new event [1346921205236] : Processed new event [1346921205431] : Processed new event [1346921221640] : duration rule with expression triggered after 20019ms