It's probably a question of how much of the heat it produces goes into heating the food, verses how much goes into heating the kitchen, and how much is wasted in heating the internal metalwork and then lost when the oven is switched off. This latter part might effect how long it takes to heat up and therefore how long in advance you need to switch it on. Mine, which is probably about 5 years old, takes about 5 minutes to get to max temperature, whereas one I recall my parents having one some 20 years ago of about the same power rating took probably over half an our to get to max temperature. That would result in vastly different energy consumption if you wanted to, say, cook a pizza for 10 mintues in a hot oven.
I've no idea which aspects of the efficiency the letters you've seen refer to though.