Getting col__ prefix for columns that are not special when extractHeader is enabled

As per documentation, Drill appends col_ to the columns that start with a number or special characters.

/**
 * Prefix used to replace non-alphabetic characters at the start of
 * a column name. For example, $foo becomes col_foo. Used
 * because SQL does not allow _foo.
 */
public static final String COLUMN_PREFIX = "col_";

But in my case I'm getting it even for all alphabetical column name.

I have the following data in the CSV file,

While querying on the csv file using following query:

SELECT * FROM dfs.`/var/lib/PRODUCT.csv`

The output is 

I know about other criterias like

#UNITS is changed to col_UNITS

FINANCIAL$RECORD is changed to FINANCIAL_RECORD

But what's with PRODUCTID; Why is it changed to col__PRODUCTID_? In this case it has appended extra underscores also.