Ha Ha! Good question.
I think the answer is that there are several "calculi". A calculus is, as the name implies, a method for calculating something, and calculi for different "somethings" are not related to each other. A partial list...
1. Differential calculus (for calculating slopes of curves)
2. Integral calculus (for calculating areas under curves)
3. Propositional calculus (for 'calculating' conclusions from premises in symbolic logic)
4. Wick calculus (for calculating normal ordered products in quantum field theory)
There are others, but I think you get the idea. By far, the most famous of these is the material contained in #1 and #2. Given the preeminence of those particular calculi, they are typically called "The Calculus" (big "C"), or simply "Calculus".