There are many city pairs that also have 'C' class for sale and this is probably one of the major reasons for the fluctuation. Remember that when most people make a J/C class booking, they won't bother with seat assignments until they go ahead with ticketing.

I am pretty sure it's a computer program that manages C class and it's pretty stupid too. For example, if a flight was an A320 (20 J seats) showing J5C0 and the flight switches to a domestic 763 (38 J seats), it'll showing J9C0 even though the size of the J cabin just increased by almost 100% and it's a day before departure (this is for a flight coming up this weekend).

Some days I think their yield management system for C class is so simple that it contains one line of code: random (0, 9) for C class so there's no logical explanation why it's C0 on a flight that's completely empty in J!