Rails decimal :scale is not working , what is the possible reason?

Question

class CreateItems < ActiveRecord::Migration
def self.up
create_table :items do |t|
  t.string :name
  t.text :description
  t.decimal :value , :precision => 8, :scale => 2 , :default => 0

What is the possible reason of when I key in value in form ,value: 192.124325345 , I didn't get a 192.12 ?

I rake db:rollback VERSION=0 before to get to the original point and db:migrate it. Maybe the rollback is not clean?