How can there be such variation in the number of calories of supermarket muffins?

You answered it yourself – sugar and fat!

The range will vary depending on the ingredients.
Some muffins have chocolate. Some will have blueberries.
Some will use recipes that call for different kinds of sugar.
Also, estimated carbs in foods baked in-store can vary wildly.

I find it hard to believe that is is the blueberries that are adding so many calories to the blueberry muffins.

Some stores might have larger products than others. You’d have to buy and compare the size of each to compare properly.

Blueberries can be a little tart so they probably add more sugar to them. The berries themselves also have sugar, more than corn does.

The differences are the total mass, mass of sugar, and mass of fat.

Carbohydrates, sugar and starch, are 4 calories per gram
Fat is 9 calories per gram
Fiber is zero, but it is usually included in the carbs so you should subtract that out.

Now do some math. Example”
Carb 67 grams
Fat 10 grams
Fiber 2 grams

Carbs: (67–2) grams x 4 = 260 calories
Fats: 10 grams x9 = 90 calories
Total: 350 calories.

Protein is 7 calories per gram but who are we kidding? It’s a muffin not a steak!

Also, assuming the same density, mass goes up as the square of the diameter and linearly with the height. A small change in both directions, say 10% (not really noticable, will increase the mass and thus the calories by 30%!

Next time I go to the supermarket, I will compare the sizes of the corn and blueberry muffins. My guess would be that since they are all laid out in the same way, that they would all be the same size, but I could be mistaken. It would require a scale factor of about 17% for size to be the only factor.

Probably fat. Look at the nutritional info. Fat grams you multiply by 9 to get calories. Carbohydrates (sugar) you multiply by 4.