yeah it is because of the 350 day rule, each day you should see another day open up.

I've heard you can ring some airlines up for dates beyond 350 days for point to point availability, not sure about RTW.

Also remember that you only have to have the first flight booked before you travel all your other dates can be open, but you need the D class availability for whenever you want to fly.

My Dubai ones on both trips were left open, just had them dated for the day I got back from my main holiday and then had the dates changed later on.