In a bottom / side freezer it is situated closer to a compressor , during the operation the compressor will generate heat,  this heat can make it difficult for the freezer to reach it temp as compressor heat may infiltrate into freezing Chamber
thus the compressor has to do more work to reduce the temp in freezer more work is more power consumption thus lesser efficency
But a top mounted freezer is away from compressor thus may not have compressor heat infiltration issues there by Increasing efficiency