Shhh...be very quiet

Some city workers just knocked on my door. I think.

I thought I heard a knock and Sheila started barking her head off. I went to the peep hole in the door and saw someone I didn't recognize in a uniform, but I still was wearing on my nightgown, so I didn't answer the door.

See, the thing is that on my door is a sign which says "If I'm working in the back of the house I may not hear you; use the door knocker and knock LOUDLY."

If this guy knocked (and I assume he did, because I really did think I heard something), he knocked very, very softly. Now. My question is--if you expect someone to answer the door, why do you knock as if you're afraid you'll wake someone up? It was 7:30 a.m., so it was logical to think that the household might be sleeping (though I'd been up since 5), but still--either you don't want to wake people up, or you do, so why be so very polite about it?