I agree that this isn't maximum "downloads" but rather maximum "devices".
I was not aware that publishers can set the limit below the standard 6 devices, but there is enough collaboration in the comments to make this very likely. If this is in the fine print, I have not seen it. From Kindle (1st Generation) Tips & Assistance
Digital Rights Management Issues
* Content from the Kindle Store: Most books and other non-subscription items you purchase from the Kindle store may be downloaded for your personal use on to up to six Kindles registered to your Amazon.com account. If you have exceeded your download limit, you must purchase an additional copy of the title. Subscription content can only be downloaded to one Kindle at a time, and only the seven most recent issues will be available for redownload in the Kindle Content Manager or from the Manage Your Kindle page.
It appears that "Most books" is an indication that some books have a lower limit.
The ability of publishers to set a lower limit appears to be unique to the Kindle Store. There is no device limit with eReader ebooks. I have never heard of a MOBI ebook having an activation limit lower than the default for that particular ebook store (some stores don't allow 4 devices, but if they allow N devices all their ebooks allow N devices). I can't see how a publisher could set a limit below the default six for Adobe ebooks, since there does not seem to be a way to tell Adobe which of your six registered devices to allow.