Why is the appeal invalid even though I posted it on time?

Why is the appeal invalid even though I posted it on time?

Your appeal is invalid as you may have posted it on time, but we did not receive it within 4 weeks from the date of the decision of the planning authority.

If your appeal is after receiving leave to appeal from the Board, your appeal is invalid as, even though you may have posted it on time, we did not receive it within two weeks from the date you received the notification letter.