What makes you think YJ-17 is an "actual missile" versus the other ones?
They all look like varying degrees of mockups to me.
Precisely because of the varying degrees of mockups. If we make the assumptions that YJ-20 and YJ-21 are both mockups. They can be said to represent a basic mockup level with minimal details. However, this is tricky on ALBM and BMs in general because they are lacking in those finer details regardless.
YJ-15 is the most obvious candidate for mockup. It can be said as a basic level mockup as well for a missile that would otherwise expose details like rocket motor, panels, screws even at this resolution we are getting.
YJ-17 has all the screws, panels and rocket motor details that if it is a mockup, would represent as yet an unprecedented level when we have at least two sub levels of mockup "detail". These are the reasons I think it's not a mockup. If it is, why are the mockup levels at three distinctly inconsistent levels. I think at most, it's just two levels of mockup detail - very scarce or with a few token details.
Now compare with the UADFs. We are given some general rumours that the two CCAs (Types C and E? these type designations have lost consistency by now) are mockups but they represent medium level. For both, you can see control surfaces details and some panels. For the two heavy UADFs they are claimed by reputable sources to be actual aircraft shown. Sensors, landing gears, wiring, panels, engines, all contribute to this.
Coming back to the missiles. The YJ-20 and YJ-21 mockups show surface waviness, commensurate with mockups. YJ-15's rocket motor is not detailed, perhaps for good reason. So my question was to ask us why YJ-17, being a more "high tier" weapon than YJ-15 is given the chance to reveal the actual missile. Assuming I'm correct. My theory is the reason is the same as that for the UADFs vs CCAs (actual presented vs mockups shown).
I think it's a messaging attempt. If these high tier ones are showing real, the lower, easier stuff are also ready. It's a meta-messaing. We can't ignore the dimension of deterrence here. The audiences aren't just domestic ones (there is intention to achieve some things with the domestic commoner). There is intention to deliver some goodies to the military enthusiast who is pro-China. The intended message to intelligence and adversarial militaries is this is ready, probably been ready for a while. We're not necessarily showing the best stuff or all the stuff. Don't miscalculate and fall under the trap of doubting these are real. We'll show the higher end ones and give those away. Lower end equipment is much less worth doubting and not showing further details is simply not giving away any more than necessary in getting the message across clearly. The Chinese truly believe in the whole "the best way to win wars is by not fighting". This doesn't involve not having the best hand in a showdown. It is to get the other guy to fold before the showdown. It's a step above proving you have the absolute nuts and in real terms, avoids unnecessary loss. If USA wants to fight, we both suffer but China wins. Better to win without suffering. Showing some stuff can cool the warhawks down in the US. If they want to chase an arms race, great, USA dies faster.
It's at this point that we ought to remember that they might have selected the 10 things out of many more to display. The selected displays are done so with intention as well. It's too fanciful to speculate there is so much more and better stuff so let's not do that now but it's worth considering why they are comfortable revealing anything with these levels of capability and tech.