Mockups carry their own config
A generated mockup usually ships with its own theme — its own spacing names, its own palette, sometimes its own font. The names collide with the real system's names while meaning something different, so porting a mockup by class name produces a screen that looks almost right and matches nothing.
The rule that fixes it is small: map computed values, not class names. Read what the element actually resolves to — font size, line height, padding, radius, shadow — and find the token in the real system that carries that value.
Fidelity is proven, not asserted
Rendering both at the same width and diffing computed styles finds the differences that eyeballing does not: a max-width, a section gap, an alignment. Those, rather than colour, are the usual reason a built screen reads differently from its mockup.