Because......
When you bleed w/ the solenoid...the water bleeds off the top of the diaphragm much slower than w/ the bleed screw....therefore.....the bleed screw may "force" the valve open...but replace the diaphragm and I'll bet your good as new...
BTW...Bill Painter is the the guy that explained that too me painstakingly about 15 years ago by calmly and patiently removing the bonnet of a valve and showing me how it worked...thanks Bill!
Love ya!
Sprinkler Solutions, Inc.
Arizona and Colorado
www.sprinklersolutions.net