Announcement of a merger doesn't mean the companies magically merge that day. The actual merger date is May 22, 2012, so just a little over a month ago. Nowhere near enough time to have a tablet designed and ready to hit the shelves.

There are pretty strict regulations for pending mergers, requiring the two separate companies to still remain separate. When Compaq and HP announced the intent to merge, some people left both companies to form a new independent company. This new single company drew up all the merger plans, which products would be kept, etc. It all had to remain highly confidential, and Compaq and HP still had to carry on as normal. Once the merger was official, that separate company was also merged in and the plans were announced. Action on those plans took a few months to fully take effect.