org-table-number-fraction

org-table-number-fraction is a variable defined in `org-table.el'.
Its value is

0.5

Documentation:
Fraction of numbers in a column required to make the column align right.
In a column all non-white fields are considered. If at least
this fraction of fields is matched by `org-table-number-regexp',
alignment to the right border applies.

You can customize this variable.