Why is coolant bleeding so difficult at times. ('03 with 3.0 six cylinder)

[Rockabilly Rebel]

My Sure-Fire way:
1- Turn key to acc., have the heat all the way up and fan at low.
2- Unplug the Heater Control Valve elec. plug.
3- Fill the exp. tank all the way to the top! All the way!
4- Squeeze the lower rad hose a few times until no more bubbles at exp. tank
5- Remove excess coolant with a turkey baster (there are 2 rounds on the level stick - top is "add" coolant, bottom is "full" - you have to bend down, be at eye-level with the top of the exp. tank and check the top of the tank compared with the height of the round.) Leave a little extra coolant, just a hair above the "full" level.
6- Replace the cap, start the engine.
7- Wait a minute, rev it at 3000 rpm and crack the bleeder screw while at 3000rpm - you'll see air bubbles, close the bleeder screw, let the rpms down again.
8- Repeat this process until no more air bubbles.
9- Re-plug Heater Control Valve elec. plug
10- Go for a good drive.
11- Make sure your heat doesn't get above 12 o'clock position. If it does: Shut Down. Let it cool down and start over.
12- Check level next morning, make sure top of the tank is level with bottom round.