You bought a yard of the EXACT same tartan as your kilt (i.e. it came off the SAME bolt) and it won't match up? Or, it's the same tartan, but off another bolt, from another weaver, different weight?
I wasn't there when the kilt or the extra yardage were cut, but either the two pieces aren't from the same run or the extra yard is the other half of a length that was split. It could be either situation: At one time they were weaving it double-width, but they stopped that soon after Mein got his fabric.