I was greatly puzzled by this too - it's the thing Ed refers to as three single phase.

The current is flowing in the same direction in all the wires in one slot.

So where you drew in the green arrow on the coil leg, it would actually be going the other way.

Another way to describe it: in the first slot the current is flowing toward the centre, in the third slot, it's flowing away from the centre, 6th toward, 12th away, 15th toward, and 18th back to 1st.

If you can imagine unwinding all the wire, you will see the current all flows around the big loop in one direction.

I took a piece of that white 14 gauge house wire and made little arrows on it for current flow, made a big loop, then did the same type of criss-cross winding. It works!

I actually went to the trouble of rewinding this tiny little heater motor armature to check this out:

and sure enough when I applied 12 volts DC to the coil, each of the poles appears opposite polarity (N-S-N-S etc) all around the armature (checked with another small neo).

If you get a chance to take a peek inside a Delco alternator, they wind their stators very similar to this.