Standard DBF format allows field names up to 10 characters. When you add table to a database container then you can define field names up to 128 characters long (these names are not stored in DBF but in DBC). AVERAGEPRICES1 is 14 characters so it must be in database.
When you copy table included in database outside (the copy is a free table) then long field names must be substituted by the standard names.
To preserve long field names you have to specify DATABASE in COPY TO command.