I thought so too. Could it be that gpt-4 turbo is more efficient for them to run, so the price is lower, but tries to maintain the token throughput of GPT4 over their API? There are a lot of ways they could allocate and configure their GPU resources so that GPT-4 Turbo provides the same per user throughput while greatly increasing their system throughput.
Unless they captured many different times and days, that is very likely a factor. GPU resources are constrained enough that during peak times (which vary across the globe) the token throughput will vary a lot.
OpenAI are doing a ton of load balancing, presumably constantly tweaking batch sizes to try to optmize across all their workloads.
You can test the GPT-4 vs GPT-4 Turbo on Playground to intuitively confirm that the speeds are similar.