The baked potato:
Nutrition Facts and Analysis for Potato, baked, flesh and skin, without salt
The baked potato that weighs 100 grams is 93 calories. (74.9 grams water)
Nutrition Facts and Analysis for Potato, flesh and skin, raw
The raw potato that weighs 100 grams is 77 calories. (79.3 grams water)
That makes sense because the water has no calories. The thing here, I think, is that you are comparing 100 grams to 100 grams, not a potato to a 'similar size' potato. As you said, some water was lost to steam. You're not comparing the baked potato to itself, raw to 'after being baked.' You're comparing two equal-weight potatoes, but one is more 'concentrated in calories' even though it is just the same weight as the other. It's more concentrated in calories because it lost weight when the water evaporated, and did not become lighter in calories because it still, for comparison purposes, had to weigh 100 grams. So it had more potato with less water to meet that 100 grams qualification, because it still had to be a baked potato - in fact, it had to be a different potato to compare 100 grams to 100 grams, one raw and one baked.
For comparison purposes, the baked potato has more 'potato' when weighed, so to speak.
This is kind of the idea behind Rolls' Volumetrics type of dieting. Make things less dense. Eat more soups and things that are watery. That kind of thing.