The table row with 6k figure refers to high efficiency, not high compute mode. From the blog post:

Note: OpenAI has requested that we not publish the high-compute costs. The amount of compute was roughly 172x the low-compute configuration.

That's "efficiency" high, which actually means less compute. The 87.5% score using low efficiency (more compute) doesn't have cost listed.

they use some poor language.

"High Efficiency" is O3 Low "Low Efficiency" is O3 High

They left the "Low efficiency" (O3 High) values as `-` but you can infer them from the plot at the top.

Note the $20 and $17 per task aligns with the X-axis of the O3-low

That's high EFFICIENCY. High efficiency = low compute.