Some threads appear to be missing. Culled?
No, not to the best of my knowledge. Something has to be really bad to be culled.
It's something to do with the software, I think.
I have noticed recently that if I go straight to a forum and miss out the top 50 list, I see a thread or two sitting there in the individual forum which I really don't remember seeing on the Top 50. This happens occasionally with things I post too - they appear on Home and in the individual Forum chosen but don't appear on the Top 50.
It's that kind of problem and what the answer is, I don't really know.
Tweak of the software, probably.