In order to speed up processing time to be able to see a preview, down sampling is applied and the the adjustment is applied to the down sampled image being displayed in studio. When hitting apply, the adjustment is being applied to the full scale image. What this means is, there could and typically is a difference between the preview being shown in studio and the output. HD Preview still has the issue as it is allowed to not down sample to the degree as the regular preview thus allowing it to be slower.
This is something that we are aware of and many of us don't like, but it really comes down to being able to quickly show a preview when changes are made in an adjustment. Depending on the size of an image results can vary.