A-weighted is supposed to give a better indication of how “loud” low level noise sounds. It is popular with manufacturers and sales teams because it is always a lot lower than unweighted (Z-weighted / zero-weighted). What it does is to pre-filter the sound to reduce very low and very high frequencies, as human hearing is less sensitive to these frequencies, especially when at low level.
There are a couple of experimental plugins for measuring A-weighted noise level - nothing “official” yet though. One of them is here: "Wave Stats" plug-in
Not necessarily. The other computer may be generating more electrical noise, and cables are only resistant to electrical noise from outside - if the noise is “in” the USB power voltage, then it will travel up the USB cable unhindered. Some USB devices are better than others at filtering out noise on the power supply, but if the USB power line is noisy them some of that noise is likely to end up affecting the microphone signal.