What you say can be a part of the explanation. I don't know exactly, because I didn't have the possibility to investigate the total blood volume of the athletes, requiring a particular use of MRI.
However, if this can be a valid reason in the case of Zhao Jing (because 3 months before she was in very little training), I don't think can be the main reason for Ding Changqin and Li Zhenzhu, who already were in deep training from the month of May. What I think is that these athletes, better than the other, were able to increase the affinity Hb-O2, so were able to bind more Oxygen than the other athletes.
About the second point, I say that Kenyans and Ethiopians training with me have in their training the speed for their event, and also something faster.
For example, Shaheen had, in one session, 4 couples of [ 1000m in 2'23" >< 5'30" of recovery among each couple, and this is absolutely a level of speed faster than his WR in steeple. But also athletes able to achieve the same performance in altitude, and using similar training, use frequently tests carried out at a speed faster than the race pace, but their final result at sea level is very different.
In this case, trying to give a physiological explanation, I can suppose the difference is in the attitude to reduce the affinity Hb-O2 when they have to bring Oxygen to the tissues.
One interesting thing, that makes me think EPO doesn't work with athletes having the physiological characteristics I described before, is that they are able to maintain the same low viscosity in altitude too, and there is no change in the heart rate for a workout of same intensity, among altitude and sea level.
For example, if I have an athlete living and training at sea level able to reach 210 HR after a test of maximal Lactic Power (for example, 300m uphill at max intensity), when I ask him to do the same thing in altitude I can see a reduction of his max HR in the range of 5%-10% (190-200). This is due to less power of the heart (which, being a muscle, needs Oxygen in order to work at max level) because of the hypoxia, but also to the increase of periferal resistances to the circulation, since the blood has higher viscosity.
Instead, when I try the same thing with one of the top athletes (I had this experience with Shaheen, Silas Kiplagat, Caleb Ndiku, Thomas Longosiwa, Edwin Soi, Christopher Kosgei, Wilson Boit Kipketer, Wilfred Bungei and many others), I didn't find any difference in their max HR between a similar test at sea level or in altitude. We can explain this fact only with the low viscosity, not affecting the velocity of the circulation.
If we give EPO to these athletes, the only consequence we know of sure is that the viscosity can grow, and this becomes a limiting factor creating for the athlete more disadvantage than the advantage he can have from the hypothetical increase in the ability to transport Oxygen.
This is, of course, not true for all the athletes not having the same type of characteristics. For example, Dorcus Inzikuru, the Ugandian winner the first edition of 3000m steeple in WCh (2005), was not part of this category, her physiological behavior was to "pay" very much training in altitude under the point of view of max HR and speed in longer tests. But, not casually, Dorcus is an Ugandian of sea level, near the border with Congo, not near Kenya..... and this can only confirm my hypothesis.