David, a page size in cm will get cropped to whole pixels as that all an image can be saved as.

There are 96 pixels to the inch and 2.54cm in the inch.
So 16x9cm is 604.72440945 x 340.15748031 pixels.

You, however are setting the dpi from 96 to 100, giving 629.92... x 354.33... px.

Instead, I would set the page to 640 x 360 (or 640 x 480), Export > Page keeping 96 dpi and in PP, scale down to 16cm across.
The resulting dpi would be 101.6. The dpi is irrelevant.

You enter 640px and will then se 6.67cm in the Custom Page Size in P&GD.

Acorn