Most airlines calculate change fees as penalty + fare difference. Anywhere in the world.

Your story is difficult to follow without fare buckets.
- Considering your fare was only c. 25% of the full Y fare, 10%/30% penalties seem very generous anyway. (Dare to say, I cannot remember that I have ever seen such a discount combined with such lax fare rules in China.) Heavily discounted or promo fares may have severe restrictions - anywhere in the world. "10% of full Y fare" could have been in the fine print.

- Maybe you booked a premium eco and when you tried to change it, the class was full?

Overall, why not book with ctrip/elong to avoid dealing with airlines directly - they also transparently show the absolute penalties when booking and you can change online, too. Heavily discounted tickets in China may have tight fare rules.