David,
As far as I can ascertain, the only "in-house" bake-off is the Deck-the-Halls, where the submission guidelines specifically state that the bake-off celebrates "the Sphere's achievements" during the previous year.
The Light Verse, Lists and Litanies, and Translation Bake-offs are open to everyone.
Also, Alex now posts this as an Event on Facebook.
And, no, Sean, no "numbers are being inflated (or deflated) to motivate people
. Gail and I have just read our 100th entry today.
Cathy