Not all 108" backing fabric is 108"!
I recently ordered a Wilmington Print Essentials 108" wide backing for a quilt. This was a replacement for some backing I had already bought that was not big enough.
When I received the fabric I pressed it to get the seams out and then I measured the backing from selvedge to selvedge. The Backing was only 104" - even though it was clearly stamped on the selvedge as 108".
I needed a minimum of 108" since my quilt was 102" long and I was loading it on my longarm. Luckily I had ordered 3.5 yards of it. So I was still able to use it - just not attaching the selvedges to the leaders.
So my advice - measure your wide backing to ensure that it is wide enough!
I did find the Wilmington Print website and sent them an email explaining the issue.