= Solution
One explanation is \b[<block-copolymer kinetic trapping>]. A solvent-swollen transient sheet structure can be immobilized when a block vitrifies during drying; accurately measured block fractions do not guarantee <thermodynamic equilibrium>. Thermal <annealing> above the relevant <glass transition>, or a controlled nonselective solvent-vapour anneal followed by structural characterization, can test whether the sheets relax toward the bulk composition-appropriate morphology.
A second is \b[surface or confinement selection of <lamellar microphase>]. The diagram describes a bulk reference model. A thin film, preferential <wetting> at the substrate/free surface or a commensurate confining thickness adds interfacial and geometrical <free energy> terms that can favour layered arrangements even at an asymmetric global fraction. Unequal layer thicknesses can accommodate $f_A=0.15$; equal block fractions are not required for every layered metastable or confined structure. Varying film thickness and surface chemistry, and comparing bulk-like and confined samples after adequate <annealing>, distinguishes this explanation from kinetic arrest. Strong conformational asymmetry is another possible departure from the reference model, but an incorrect measured composition need not be invoked.
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