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It's the result of something bad, even dangerous. That voltage drop means that there is excess resistance somewhere in the circuit. It is a potential fire hazard because heating is sure to occur where this is happening. The most likely culpret is a bad connection like a loose screw or rivet on a connection somewhere and it probably is in the house or in the breaker box, not beyond. A likely candidtate is the socket where the roaster plugs in. Run the hair dryer because it doesn't mask odors like the coffee roaster would, and search the house for any heat smell, roasting plastic or burning smells. Check inside the house at switch plates, outlets and the breaker box. Don't run the circuit very long until it is fixed. Plug the roaster in a different outlet and measure the voltages there. Tell the manager and insist that an electrician be called to look into it. Even if you find the problem, don't fix it yourself. Insist that a licenced electrician do it so you don't void any insurance should there be a fire.