Dan,
I just took a look at your file and am not seeing a problem unless I drag it almost off the paper. One piece of info that may help: dimensions are designed to detect the scale of viewports automatically, based on the location of the first point of the dimension. If you are using viewports on your drawing and move the dimension to a different viewport, or outside of a viewport completely, you will see the dimension change in value. It may seem kind of weird, but it's designed to help keep the dimension accurate.
Bill Myers