If I were to through a guess at this, I would assume it is because UE4 tries to adhere to variable naming conventions of camelCase, which for many programming languages starts with a lowercase (not with standing PascalCase, which starts with uppercase) and capitalizes the first letter of each word after. As in the case of UserAccount, in javascript for instance, the convention would be “userAccount”. It is worth noting that JSON does stand for javaScriptObjectNotation.
I am not sure if I would agree with UE4 forcing conventions on coders, and I could be completely wrong.
You probably just need to adjust for the lowercase letters on the receiver.