why the discrepancy in drain hole size for Roltechnik Hawaii-P shower tray

Question

I have an 18+ year old Roltechnik Hawaii-P curved shower tray with the following specifications:

Model: 900MM
Dimensions: 900x900x170 mm
Drain Hole Size (as per specifications): 60 mm (Φ60)

However, after removing the strainer, I measured the actual drain hole and found it to be only 50 mm in diameter.

I am confused about this discrepancy. Why does the product specification indicate a 60 mm drain hole size when my measurement shows it to be 50 mm? Additionally, I noticed that the only other variant available is the 800x800x170 model, which also lists a 60 mm drain hole.

Is it possible that there was a change in design over the years, or could there be a specific reason for this difference? Any insights or experiences with this product would be greatly appreciated!

EDIT: Here are images from 2 sources of the same shower tray type:

Answer 1

60mm would be the outside diameter of the strainer - that provides the lip to stop it falling through the 50mm hole.