"It's just a computer program. It must have followed a particular series of steps and decision points?"
Imagine the data is a photo and the program is a fancy filter effect. The filter is just maths: it produces the new pixel by combining pixels in the old image.
Now try asking why the filter looked good on one photo and bad on another. For each pixel in the output we can say it added 1% of the pixel to the left, 2% of the pixel two to the left, and so on, but that doesn't really help us understand why the first photo looked good and the second looked awful. It's the interaction between data and "algorithm" that produces the result by the magic of arithmetic.