Help me out understand a CRC residual

[copper230230]

Dear Sirs,
When I study the USB spec, it said that the CRC polynomial of token was:
G(x) = X^5 + X^2+1

When the host send message with crc to device, then device use this CRC to
check these data. If no error occured, the residual was X^5+X^3+X^2.

I also found some document, but I can't understand what it meant at the item
3 at page 2.
( www.usb.org/developers/whitepapers/crcdes.pdf )

Why the residual will always be a constant?

If someone can kindly tell me the math inside this constant which CRC
residual.
Thanks!!

 

[kelvin_sg]

Help for CRC

If you invert the received message and run through the CRC checker, you get residue of some fixed number.
If you don't invert, you get residue of 0.