There are a number of factors that will affect the yield:
Initial product condition
The initial quality of the product prior to thawing will effect the yield of the product after thawing. Product that is not in good condition or has been incorrectly stored prior to thawing will have a lower yield, requiring extra trimming or down-grading.
Final thawing condition
If the fish is not thawed completely and the area around the backbone is still surrounded by ice, it will be very difficult to remove the fillet completely (as well as being very dangerous). This will result in a lower yield. It is better if the fish is just thawed near the backbone as it is easier to remove the fillet completely and maximise the yield.