See the second screen shot of a form on this page : http://www.open.edu/openlearnworks/mod/oucontent/view.php?id=52747§ion=2.1.3
If you just filled out predefined values for variant 1 then you would only have one question variant essentially you would have a question that functions the same as a variable numeric question type.
"So I could put an expression such as this vol=rand_int(50000,99999)/999.9 as the calculated variable and just leave the variants blank"
Yes. As with the variable numeric question type the value of that variable will then be calculated.
Question variants are the same question but with different sets of predefined value variables.
Have a play with the question types on your Moodle installation, use the question preview feature, hopefully all will become clear to you.