This is actually from the
manual for the 120e
notice the circle with the + and -, that's a power supply. Connecting the + side to the c terminal seems to be their preferred approach, but it states "5-30VDC Non-Polarized" which means it should work either way. Opto-isolators typically only work in one polarity, so generally you may need to pay attention to polarity. Some types of digital inputs require pullup or pull down resistors, but not opto-isolated which actually depend on a small amount of current flow instead of simply measuring the voltage.
As to what you observed, I think it's a result of the way they chose to make it work with either polarity. However, I would switch it to the + to com configuration they recommend. I don't think there's resistance inside your PIR, most are a simple relay like an on off switch for their output switching the com terminal between normally closed and normally open.
I also shouldn't have referred to the negative lead the way I did (edited now, but it was the middle of the night), it implied a negative voltage instead of ground or 0VDC.
Also note, some models of sealevel products have different types of inputs.
