I see a whole bunch of candidates here, each backed by a broadly plausible psychological story:
1. They are more ruthless than we realize.
2. They are more like us than we realize.
2b. #1 and #2.
3. They have longer time horizons than we imagine.
4. Due to extreme political constraints, they have far shorter time horizons than we think.
5. They are more inured to the risk of economic depression and hardship than we grasp.
6. They are more obsessed with parallels to earlier Chinese history than a typical Westerner would find natural.
7. They are less rational than social science rational choice models would predict, having one or two major blind spots on matters of critical importance.
8. The Chinese see themselves as weaker and less stable than we see them.
9. All of the above.
10. Good luck.