These numbers seem to roughly map DRM "security levels" and allowed resolution levels. Basically, on platforms that support hardware Widevine (or some other DRM product) ie. "strongest security", 1080p+ is allowed, as there is a low risk of the warez Scene™ being able to tap the decrypted media – HDMI splitters that can strip HDCP 2.2 are hard to come by.
On platforms with only software DRM (tl;dr an obfuscated binary blob distributed along your browser that does some form of AES decryption), only low resolution streams are available because there is a good chance some folks somewhere have tooling to intercept the decrypted media.
On platforms with only software DRM (tl;dr an obfuscated binary blob distributed along your browser that does some form of AES decryption), only low resolution streams are available because there is a good chance some folks somewhere have tooling to intercept the decrypted media.