Why wood moves across the grain
Below the fiber-saturation region, wood changes dimension as bound moisture changes. Tangential movement is generally larger than radial movement, so flat-sawn boards often change width more than quarter-sawn boards of the same species.
What the result is good for
Use it to sanity-check gaps, panel expansion allowances and seasonal fit. Do not use it as a structural design value or a promise of an exact future dimension. For high-value joinery, measure the actual material, understand the expected environment and leave a construction detail that tolerates movement.