If the appraiser had the stone loose that's awful.
If, as I suspect, the diamond was set in the ring this carat weight difference does not reflect poorly on the appraiser.
If you read the appraisal carefully the specs being only estimates (since the diamond is not loose) is probably in there.
If you can't weigh the diamond alone you can only guess at the carat weight, so it's harsh to say 'the appraiser got the weight wrong".