You don't say what behaviour you were expecting!   :)

By design, almost all Decimal operations (but not creation of a Decimal from an integer or string) round to the precision given by the current context.  By default that precision is 28 significant digits.  See the documentation, particularly the quick-start tutorial,

http://docs.python.org/library/decimal.html#quick-start-tutorial

for more information, including how to change the default precision.