IIRC, in the previous generation, the main reason 3700X had 65 W TDP as compared to 3800X was the assumption of a much worse cooler being used. I'm not even kidding. They were saying you can use a worse cooler because they chose to use a worse performing cooler in their calculation. The calculation isn't based on actual power limits or anything of the sort. I'm guessing it's more of the same here. No wonder they're aimed at OEMs. Essentially, it's a license to use crappy coolers. It allows you to build a computer with worse cooling and you'll pay for it with worse performance.

It doesn't mean the processors are more efficient (at any given frequency). Higher efficiency would require better silicon and that would cost more money. It's reasonable to assume that if one version has better silicon, it's the X. As I expect it to be more expensive.