Could be any reason. The formats aren't the same, especially if you want to make full use of the PDF format (bookmarks, hyperlinks, etc.)
Or maybe they want a simultaneous release.
Here is the actual update from Monte so I don't make any mistakes:
"Things are going great. The corebook goes off to the printer in a few days, which means everything is on schedule. Barring unforeseen circumstances, we should start shipping orders the first week of August. Signed books will probably go out the second week in August, just due to logistics. Numenera Thunderstone should ship in August, but it might be late August. That will be coming from Alderac Entertainment Group.
To be really clear, backers who ordered the corebook, the deluxe corebook, the Numenera Player's Guide, the Nightmare Switch, the XP Deck, the Cypher Deck, the GM screen, t-shirts, and the dice should get them in August. I can't guarantee a precise date because of the vagaries of shipping, but all of these things should be shipping early August.
Ebooks for those products should go out right away at the beginning of August, as should the paper terrain ebooks from Fat Dragon Games."
It sounds like they are going for a simultaneous release. Beginning of August is the release date, it just takes the books longer to arrive.