Why 16, 18 mm ?
 

Many of the old bolts on my car were 17 or 19 mm. Now, the replacement bolts I'm getting are 16 or 18 mm. For example, the parking shoes come with 16 mm bolts for the brake calipers.

Why did they make this change?

New style, keep the tool companies in business??

This might be a long shot, but 18mm is a popular size for Chrysler vehicles. Maybe Daimler and Chrysler are trying to commonize some sizes.

Are you getting genuine German bolts?
 

For as long as I can remember (>50 years) the European car makers have stuck to a 8, 10, 13, 17, 21 set of head sizes (occasionally a 12, but rarely) unless there was a serious reason for something in between, but the Japanese cars and other mechanical devices have often also had 9, 11, 12, 15, 16, 18, etc, along with a tendency to use many more fine and coarse pitch variations than the Europeans.