Why is it when a return is extended, the letter changes to say taxes are not due until October 15 (or September 15 for (LLC's & S-Corp's).

All taxes are due by 3/15 or 4/15, and the extension does not change that. The taxes paid after the fact are late, and when the letter shows 9/15 or 10/15, I have to always change the letter or explain to my client that the letter is wrong.

Any ideas?