004 - people don't know how to pass.

why don't people who "pass" maintain their speed??

i'm driving on the #1 highway. westbound this time.
there's a car from novascotia with 2 ladies. one is on a cellphone.
they pass me. ok. in front of them is the semi-truck. & instead of passing the truck they slow down!! they return to the rt.lane. then they don't pass the truck. ...???
the truck velocity is slower.
i am faster than they are now. this goes on for another 30sec. now i decide to pass both vehicles. after i pass the truck they pass me again. so i give them my usual dirty/digusted look but this time they accelerate. too bad they make nova-scotians look bad.

2nd instance. same day.
a couple in a red aveo?/suzuki? (AlbertaPlates) pass me; then they slow down and force me to pass them. i stare at them.... Later on (5 minutes later,) they pass again but this time they don't slow down.

So here's the point if you are driving faster than someone, pass them and keep going.
don't pass them and then slow down to a speed less than the person you are passing.
not only is it aggravating. it is dangerous.