The Malbim points out that in the wording of Haman’s letter, Haman had left out the detail that the attack would occur in every state so that the governors would not know yet that he planned to have a mass genocide. Haman did not want them to know that other governors received the same orders. Thinking their orders were unique, the governors would focus on the work at hand, and not be unnecessarily concerned and overwhelmed regarding the success of the operation.
The Malbim writes, using the answer from the last post, that Mordechai here is showing the copy of the letter, and relating his interpretation of it. He adds that Mordechai could only get the summary version, not what the governors had. He had no other option than to mention the destruction left out of the actual public letter.
Later in the story, when Haman’s decree is rescinded (see 8:11), Mordechai advertised this fact in all the provinces. According to the Malbim, the governors of the states receiving copies of Haman’s decree did not know that other governors had the same command. This way, they were compelled to be more careful to kill the Jews under their jurisdiction, under the mistaken impression that they were alone in this task and thus under the royal microscope.