Programming language used to interact with SQL Server databases
I don't have that particular flavour of AdventureWorks, and the AdventureWorks databases I have, sum of LineTotal appears to match SubTotal for all orders, at least within the currency unit in question.
There is a moral here of database design: The column SubTotal is redundant, and with redundant columns there is always risk that you get inconsistencies due to bugs, because someone forgot to update the header table. (And here I have a production scenario in mind, not a demo database, where the reason for the inconsistency may be sheer sloppiness.)