Another way is, where feasible, use multiple raters.
If you break it down to a four pount scale, "above expectations", "meets and occasionally exceeds expectations", "meets expectations minimally", "Does not meet expectations", that will help with rater confusion. You can, if necessry, put a hard limit on how many can be "above expectations" without review by HR or other appropriate source.
You can also continue doing what you are doing but also do a forced ranking on each dimension if the groups are small enough. "Please rank from greatest to least performer on the following dimension: <e.g., quality of work>."
Use the forced rankings to put the other things into perspective. At some poine, you can take the forced rankings back out when they get the idea.
"How can your lowest ranked person be constantly exceeding expectations?"