Patterns aren't always printed exactly straight with the grain of the fabric so it could be tricky to get it to look that way.
I often use border fabric that has a specific pattern that has to be fussy cut right on a point in the pattern. If I wasn't doing that and just layed the fabric how it wants to lay and cut straight, it wouldn't always be straight. I have to lay my 24" ruler along the line I want and slightly pull the fabric up or down to line it up.