Things do change, but the OMB knows that.
One of the key things that the OMB does is make historical numbers comparable. They often go back and normalize the numbers when an accounting change or other type of change occurs.
Historical comparisons are very important and the OMB knows this. They do go back and make the necessary changes so that an "apples-to-apples" comparison can be made when a department, agency or the whole government itself changes something to cause the numbers to be altered (or accoutning differences and changes) from previous years.
Case in point: you can look at the historical data tables and see agency breakdown for the, e.g., Dept of Energy, even before the agency was created. What they did was breakdown historical spending in those years before (e.g.) the Dept of Energy was cereated and realigned the categories and numbers so historical patterns and their integrity can be maintained.
Your theory is correct, but you failed to know that the OMB takes great pains to make sure that historical comparisons are kept intact after changes to the system occur or the law changes.
I agree, though, that we should be spending MORE on Defense, not less. These proposed base closures are the wrong way to go...I hope President Bush rejects the whole lot & closes the BRAC instead...and, he could throw in the Dept. of Education, too. I'm sure Reagan--the Master of Better Living Through Increased Defense Expenditures--would approve...