In most areas of the natural, physical world, math does rule. But you're talking federal budget numbers over time here... and economics! Federal budget bureaucrats & bean counters have been manipulating math all along...and mesmerizing the masses AT LEAST since I studied the Keynesian model in the mid-Sixties. Example...
A potential problem (my opinion) with your numbers is that back in the early '60s SOCIAL SECURITY expenditures were NOT considered part of the Budget (not "spending"). Back in those days the Feds still had most people convinced that SS was not spending at all but, rather, money stored in their "SS Account" for them to draw on in retirement.
As more people became better educated in the 50s, 60s, and 70s and SS was exposed for the pay-as-you-go Ponzi scheme that it is. Congress BROUGHT SS INTO the overall Federal Budget sometime in the late 60s or early 70s and, needless to say, this throws all sorts of calculations (and comparisons such as yours) all out of whack and squarely into the realm of Federal Budgetary 'Voodoo' mathematics...
A better indicator would be Defense Spending as a % of GDP over time...I think...
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.