Budgeted Cost

In my Project Schedule i have addded only Material and Non Labor resources. I am able to manually edit the Budgeted Cost field in the Activity Deatils tab. However since this is a entity that is calculated by multiplying the Budgeted Units with the Price/Unit then how is it that i am manually able to edit the Budgeted Cost field and the Budgeted Unit field do not change correspondingly.

For example:-

I have Pipes as a resouce with a Price/Unit of Rs.2500/meter.

I assigned this resource to an activity " Laying Bridge Rack CHW Pipes" and i estimated that 150 meters of pipe will be required for this activity.

So the Budgeted Cost = 2500 X 150=Rs. 375,000/-

But after i do this how am i still amble to manually edit the Bidgeted Cost field and then why does the Budgetd Units field remain unaffected after the change is done to the Budgeted Cost.

Am I  missing a setting ????????????????????