Arduino Uno R3 with MSO-19 Scope

[oldy99]

I have the Arduino plugged into a USB socket on my Acer Aspire netbook and the MSO-19 into an adjacent USB socket. Before connecting the probe ground to the Arduino Gnd, I measured the voltage from probe ground and Arduino Gnd and noted that the MSO-19 ground was about 0.3 volts more positive than Arduino Gnd. I suspect it has something to do with how gnd and ugnd are treated in USB circuits. In the Arduino there is device between ugnd and gnd whose symbol looks like 2 half moons back to back. Don't know what that is. I also don't know what the USB circuit that Link Instruments use in their MSO-19 looks like that would account for this difference. Can someone out there help me understand what is going on? I feel quesy about connecting 2 points of different voltage together.