I have to clarify this post of mine and say that I'm not so sure this is normal. And I was so sure of myself.
I conducted some experiments, and wasn't able to get a significant voltage drop from my old 210 watt Weller soldering gun, or a decent sized fan on MY circuit.
I did get a 3.3 volt drop with my vacuum cleaner which I'm sure draws a helluva bigger load than your hair dryer, unless you're talking about something like this:
Hair dryer on high heat is usually around 1300W. They're generally sized to be as powerful as possible without tripping a 15A breaker plus a little safety margin.