I don't understand this.

Question

I have a graph in my American history and it is labeled on the side "1995 Dollars in Billions"
In 1965, it has $264 and in between 1965 and 1970, it is at $352 and then in 1970, it goes back down to about $264 again.
Then the question is:
What event would explain the spike between 1965 and 1970?
Help??
Also, why does the budget change from year to year???