Why we have to maintain 90 ohm impedance for USB signals and 75 ohm impedance for RGB(video) signals?

---------- Post added at 11:34 PM ---------- Previous post was at 11:32 PM ----------

it all depends on Cable or device connected to USB,due to high-signalling rate impedance of cable or device is generally kept at 90ohms,anyhow the range is from 70ohms to 110ohms,similarly with RGB.