Awning Sizes Help Please

Hi I think I posted this in the wrong place so hence I am posting again.

I have a Bailey Senator Montana 2003 twin axle

In the Bailey hand book awning size is 1042 I want to purchase a Trio sport Mexico but the sales man said in the book the size awning for my caravan was 1062 he agreed that in his bailey hand book the size was 1042 same as mine. I tried the string round the caravan and the size then was1048 so my problem is what size awning do I need.?

 for the size in the hand book I need a 1050 but for the trio size I need a 1075. can any one explain why the trio size is  so different to the handbook size.any help would be appreciated