My guess is that it's because you're not including the additional precision your [calc] expression is generating. The result preview is rounding for clarity.
I don't think i am actual familiar with what you described, can you provide an example of precision setting you are referring to? Thank you!
Pretty sure it is because Alteryx visually rounds to 6 decimal places. Meaning it shows 0.032877, but the true value retained in the [calc] might be 0.032876711038961038961038961033896 = 10.126027/308.
Hope this makes sense,
MSalvage