the height of the cannister inside. This is deliberate. If it were line-to-line as you secured the lid on...there would be no pre-load and oil pressure within the cannister would push by the two end seals. As you tighten the carriage bolt the corregations of the oil filter will compress if not distort slightly which is fine...this will ensure that the two end seals will not cave to oil pressure and all oil flow will past through the paper element. A further side bar to help those change their oil on their M-30...first thing you should do as you loosen the cannister carriage bolt is not pull the lid right off as you remove the bolt. First remove the bolt which will create a leak path down into the crankcase thereby when you remove the lid you won't spill oil all over.
HTH,
George