Yeah its a bit random mate unfortunately, In my opinion and again this is just my thought I don't have any actual evidence.
They get you to fill out that form to see if you meet the HKCAD CPL requirements, perhaps you were short on night or something? Anyway, once they have got lets say 50 people in London that are suitable for the HKCAD they will then select 30 most suitable to interview during the one week they will spend there.
The first time I got sent the form I sent it back and didn't get an interview, I knew people who did get an interview in that round, they generally had more experience than myself and had applied before me.
The second time I got sent the form (some 8-10 months later) I filled it out along with paper work required and got the interview this time, however I did have over 1000 hours at that point.
Regards and good luck.