The problem is something that Aerotech can not really control (any manufacturer can have this problem).
When they order cases, and liners, they are garunteed to be + or - a certain amount in spec. What happens when they order cases and liners that are in spec, but the cases are on the - side, and the liners are on the + side. They won't fit. And since they order them in lots of thousands... they have to use them because they are within spec, and they cost to much to replace.