The green/blue status bar says 20% complete because 2 of my 10 subtasks are complete, but my parent task is only 4% complete timewise. Timewise is what matters. Who cares if I have 49 tasks of 50 complete if the 49 complete tasks are 1 hour each and the uncompleted task is 2,000 hours. So I have 49 hours out of 2,050 hours, or 2.4% done. But JIRA tells me I'm 98% done.
I'm highly surprised JIRA has been around so long and hasn't resolved this silliness.