I do appreciate Michael's point that the frozen food companies expend millions of dollars to get one item on the shelves, and they have test kitchens that work to optimize the heating of the dish.
And their labs have banks of ovens.
Unfortunately, there are ovens, and then again there are ovens. Right now we have a Viking oven fueled with propane (would never buy another for home cooking). For two years of our early marriage we had a very old, inespensive gas oven. For the rest of the time, 22 years, our stoves were electric.
All of them behaved differently.
Can only imagine the food company labs fire up their stoves, probably electic since that is what most people cook with, in the morning and keep them humming until quitting time.
So, already being hot, they willl retain their heat longer and better, when opened and a frozen dish put in, than an oven rapidly brought to the proscribed temperature will.
When to toss in the frozen dish?
I don't think there is an answer to this question.
We are still learning to deal with our oven and would never put a cold or frozen dish in it at the beginning because we know our stove a bit.
But y'all are cooks and know what you are doing.
Sometimes make dishes for others, and we do not know what their stoves are like.
But we try to muddle through somehow.
It is your stove and your food, and do what you like and it is fine with us.
Take care and God bless.