What is meant by "to change proportionally less" here?

Question

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Coal prices are assumed to change proportionally less over time, but follow the direction of oil and gas prices.

Answer 1

that means coal prices will stabilize over time and won't fluctuate as much as it does now, and that it will follow oil and gas prices too (mostly these prices are stable and only increase due to high demands and shortage of oil and gas).

hope you understand :)

Answer 2

That means if oil and gas goes down, so does coal, but coal will not drop as much as the other two. .