It is about the "nonce pattern" that excludes certain ranges of nonces out of valid blocks. So there are ranges of nonces that are not likely to be found at time of the writing of that particular post.

Why? The reddit user suspects it has to do with a bitmain bug that excludes these nonces from being used. There are "unknown" entities that do produce ocassionally a block that includes a nonce in that region. Around 11%. It can be a "hardware stealth test" but it cannot be exploited.

Idk if it is still happening, but it gives some information about the workings of mining.