The behavior of this operator is determined by the data types of the columns - had they been numeric values, the + operator would have added them.
Each column has a name, a data type, an indication of whether it can contain null values, and in some cases an indication that the columns is used as a primary key (PK) or foreign key (FK).
Note that there are a few tables in the dbo schema, but most of the tables are defined in a schema named SalesLT.