Confirming serfty's comment above, yes, there is different availability in different markets. As an example, I was trying to get D class seats LHR-SYD on a weekend and one site showed D9, but my TA (airline?) could not access them. Likewise, when they rang the operating airline and begged, the answer was still "not available".

Basically, airlines are only keen to issue D seats if they cannot sell them for real money.