Cost ? The mock up constructors didn't gain actual access to build the mock up? Mis-information?
Could be all above or none of them.
All good ideas, but we've had enough pictures of what YJ-12 looks like from actual test articles (see first two pictures of my post 160) and other sources to know that the mock ups are quite accurate in that respect. (What they lack are the tail/booster extensions seen on the test articles). So the likelihood of it being your suggested answers are unlikely.
In that case, it becomes not a question of "are the mock ups representative of the actual missile" and "why could the actual missile feature these design features".
There are other missiles which feature similar design intake "extensions" btw: XASM-3's air intake structures extend all the way back to the nozzle as well, as well as the GQM-163 target drone, and the aforementioned Kh-41, and Kh-31 as well.