I assume because Ford reasoned that any time you can cut manufacturing costs and mechanical complexity without affecting reliability or performance, you should do it. So they deleted the "extra" pulley as being engineering overkill.
If you got the kit with the extra pulley you must use it.
The kit you got is for the ZX2.
If you set the upper pulley next to the one in the kit you'll see the one in the kit is slightly smaller. This is the reason for the second pulley. It takes up that extra slack in the belt.
So, use both pulleys or risk timing jumping from a loose belt.
I'll check the size on the 2 upper pulleys. When I did the belt on my car I didnt use the lower pulley and it has been fine. I'll use it on the motor I'm working on if the uppers are different. It does say "not used on all models" in the belt diagram though.