AMD has launched two new members of the Ryzen 5000 desktop processor family in the forms of the Ryzen 7 5800 and Ryzen 9 5900. Both CPUs have lower TDPs than their similarly named Zen 3 counterparts, and the Ryzen 9 5900 offers an average of 24% faster gaming performance at 1080p than the previous-generation Ryzen 9 3900.
https://www.notebookcheck.net/OEM-only-AMD-Ryzen-7-5800-and-Ryzen-9-5900-desktop-processors-bring-extra-gaming-performance-with-a-lower-power-requirement-to-the-Zen-3-family.514810.0.html
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.