Hello
Trying to explain it fairly simply:
Various 3d-Software packages and file formats do not support regular measurement units such as "Meters", "Feet", "Inches" or "Millimeters". They just use an abstract "Unit" that you can interpret as anything from Nanometers to Lightyears, if you wish.
When you upload such a file to Shapeways, the system asks you what actual measurement unit your file's "Unit" represents. IIRC, it offers a choice between Meters, Inches and Millimeters. Shapeways uses this to convert the File-units into the actual dimension of the object and to control the printing so that it comes out the correct size.
On the other hand, the display page only displays dimensions in Inches or Centimeters. The system automatically converts the unit you entered when uploading. If you entered "Millimeters", the numbers of centimeter will be 1/10th of the FIle-Units, the INches will be 1/25.4 of the File Units.
An Example:
You want something that is 5 cm long (not quite 2 Inches)
You design it in millimeter units, so it's 50 "Units" long.
On uploading, you enter "Millimeters"
The printed object is then printed 5 cm/50 mm long.
On the website you see "5 cm" or "1.97 Inches"
- Everything is OK
If the numbers of cm/Inches on the website does not corresponds to the size correctly converted to cm/In, you have a problem and its time to ask for support.
I hope this helps
Greetings
Karl Heinz