This is a known issue with Claris CAD 2.0v1. It happens only on some ruler settings. There is an issue of rounding the numbers correctly to the number of digits of precision that you are using. There are two workarounds.
1. If you have a ruler set at something like 1 = 10 and you are showing 5 digits of precision, change the ruler to .1 = 1.
2. Display less precision.
This is fixed in 2.0v2.
If you are using 2.0v2 or later and still have difficulties with incorrect dimensions, there is another known issue which may be the cause. Some CPU accelerator cards come with INITs which can intercept numeric calculation calls from Claris CAD to Apple's SANE code. Contact the manufacturer of your accelerator card for more information about this compatibility issue.