HELP!!!!

leebryant021280

My caravan is a Stirling europa 470 2000my.  I am after a full size awning and have looked on a number of awning websites where you can get the a measurement. My van is listed as having a measurement 885 but require an awning that is 875cm. Why is this?
Also if I do need an 875cm one do I get a 850-875 size9 or size 10 875-900cm.

sorry for all the questions, new to caravaning and have only used a porch awning.

thanks

lee