It is misleading to name the option "precision" when it actually refers to the
numbers after the decimal point, which should have been "scale" as it is declared
in the Entity for doctrine.

and generally in mysql :
http://dev.mysql.com/doc/refman/5.1/en/precision-math-decimal-changes.html

M is the maximum number of digits (the precision)
D is the number of digits to the right of the decimal point (the scale)