YES! There is a difference. Keys must be typed exactly as specified. The transform will not work otherwise.
I don't know why there is a difference between the keys that start with an underscore ( _ ) and those that do not. There is nothing in the documentation about that. My guess is that the difference is that the _ is used for metadata fields that are not stored in the journaled rawdata. Even if I am right, there is nothing that you or I can do about it, anyway; some keys start with _ and some don't!