Not to sound like such a sheep-but no. It doesn't say why there should be such a heavy opening dose. Nothing that caught my attention anyways.
The reason for the high dosage after a water change per Seachem is:
Thank you for your question, -------. Because carbon can be removed from the water upon performing a major water change, we recommend increasing the amount of Flourish Excel dosed on water change days, simply to re-constitute the water with carbon for your plants to utilize as needed. We hope this information helps, and thank you for using Excel!