Calling a TO to kick FG with X seconds left..

I know this is a minor issue, but it drives me mad: why the hell do the Texans always leave so much time on the clock when they kick the last FG of the half/game?

Usually Kubiak/Schaub will call a timeout with five seconds left -- I've always thought that was a few seconds too early, but it seems to generally work out just in time. Is five seconds the unofficial time it takes a FG to be kicked? When does the clock stop -- when the ball hits the net, or falls to the ground?

Just now at half, they decided to take a timeout with 6 seconds left and left a second on the clock. That just seems so stupid to me? How hard is it to wait a few extra seconds to call the timeout?

Again, I know it's a minor issue, but I just don't understand it..