https://www.w3.org/TR/css-grid-1/#algo-spanning-flex-items

if the sum of the flexible sizing functions of all flexible tracks spanned by the item is greater than zero, distributing space to such tracks according to the ratios of their flexible sizing functions rather than distributing space equally

Distribution of free space in “Expand Flexible Tracks” has special behavior for when the sum is less than one in order to get continuity between sum = 0fr and sum = 1fr, but this behavior is not reflected in the flex-dependent distribution rule under Resolve Intrinsic Track Sizes.