Yes indeed if your phone is connected to the computer not necessarily in order to sync, it counts it up. You can easily check by looking at the blue bar with the count on it, unplugging the phone and waiting for the change. Then plug it back in. It confused me as well because I plug my phone onto a desk stand for charging so I hadnt noticed. One day the office internet was out and I shared my hotspot to my laptop and noticed the count of 2.
You might also have a device lying around that you might have forgotten about, like an iPad in your car near your window or an old phone in your desk drawer. That happened to me once :-)