Because oil & gas suppliers only ever sell one product, and memory fabs can dynamically switch product mix in response to supply & demand to optimize profits. The same sand, power and water can make DDR4, HBM or DDR5
I'm not sure I follow, varying range necessarily implies varying ratios (e.g. a product missing from the range means its ratio is zero).
Even when in theory you can obtain some higher quality products, the composition of the crude can make it too complex and expensive to practically obtain them.
You don't want to refine gasoline from heavy crude, especially in winter when demand is lower. For gasoline or kerosene you want to start from lighter crude. Same with many undesired components (either from the crude or resulting from the refining methods), the more you have, the more complex the refining, and the resulting ratio of products you obtain varies.
So in practice what you get out of the refining process absolutely depends on the characteristics of the crude, and many other things like market demand or the capability of your refinery.
Same as with silicon. The process to make the wafer results in different quality if you want to make low tech or cutting edge semiconductor products.