I noticed that, too, so I read up on it earlier tonight. Apparently, hubs with an "H" next to them are featured and suggested to search engines because of their great quality, while hubs without the "H" are floating in a limbo of sorts until the quality assurance algorithm deems them worthy of additional exposure. You can try editing the hubs that don't have the "H" - it forces the QA algorithm to reevaluate them as it would a new article.
The information I read did say that not all hubs without an "H" are necessarily bad, and that not all hubs with an "H" are necessarily good. What this tells me is that they're probably still working on the new system, so I wouldn't worry about it quite yet.
Hope that helps!