Something like this: The smaller size of the cabin, and the unlikelihood that any single ticket-buyer would want more than 4 First tickets, makes it unnecessary for the CRS (reservation system) to determine precisely waht inventory is when inventory includes 4 or more seats remaining for sale.

There may also be a competitive intelligence issue here: CX would just as soon prevent its competitors from finding out its cabin loads, flight by flight, cabin by cabin, booking class by booking class. By obfuscating its data in this way, CX's competitors can't get as reliable information about CX's first loads -- can't tell whether CX has sold 0 tickets in First or 8. Yes, they'll still be able to tell whether CX has sold 8 versus 9 versus 10 versus 11 versus 12 (all assuming no overbooking), but it's not obvious what CX could do about that. In contrast, there's no particular cost to CX of hiding precise inventory when inventory is 4+.

As an economics student, I'm always on the lookout for good sources of data -- once wrote a major paper on the effect of Amazon.com's recommendation systems, measuring sales changes by changes in books' sales ranks. (Recall that Amazon publishes sales ranks for all its top-selling books.) If CX published full inventory data I could do a similar project on CX -- not sure what trends I would look for, but I'd see what I could see, and I'd have all their sales data (though only a proxy for revenue, as indicated by booking code). CX almost surely prefers that I not do this -- they'd rather commission any such analysis, not have it done by some random member of the public -- yet another reason for them not to provide more specific inventory data.

Hope this is helpful & interesting!