Centimeters are wrong
For various reasons having to do with matching up grids between different units like inches, centimeters, points and so forth, older versions of PowerPoint don't use the standard 2.54 cm = 1 in conversion factor when converting between centimeters and inches:
Real world: 1 inch = 2.54 centimeters
As reinterpreted by PowerPoint: 1 inch = 2.4 centimeters
PowerPoint is off by approximately 6%
PowerPoint Centimeters Different From Actual Centimeters explains the reasoning behind this in more detail. The same explanation applies to
- Microsoft PowerPoint 2000, 97, 7.0, 4.0 and earlier (Windows)
- Microsoft PowerPoint 98, 4.0 and earlier (Mac)
Later versions use the correct 2.54 cm to 1 inch conversion factor.
[Special thanks to Brian for this additional information]
PowerPoint 2001 (Mac Classic) and PowerPoint X (Mac OS X version of 2001, also released in 2001 after PowerPoint 2001) both use the correct centimeter conversion of 2.54 cm = 1 inch.
I created a slide on PowerPoint 2001 set to centimeters in Mac OS 9, and it preserves the size when opened on PowerPoint 2004 (Mac OS X), also set to centimeters (set to Metric in OS X). But when I opened the same one on PowerPoint 2000 (system set to Metric), it shrunk the size as predicted.