= Solution
The larger value for the carbonyl bond means that its dissociation costs \b[$167\ \mathrm{kJ\,mol^{-1}}$ more] for the supplied comparison. A carbonyl combines a strong <sigma bond> with efficient pi overlap between spatially compatible C 2p and O 2p orbitals. This gives a substantial localized multiple-bond contribution.
The short P–O linkage in a <phosphine oxide> is also strong, but it is more polar, with a substantial $\mathrm{P^+{-}O^-}$ contribution. P 3p and O 2p orbitals have less favourable radial and energy matching than C 2p/O 2p for a simple localized <pi bond>. <oxygen> donation into P–R antibonding orbitals adds stabilization, but it is not identical to the carbonyl <pi bond>. Therefore \b[the formal double-bond notation does not imply equal bond strength]. A substantial P-3d contribution is not needed to explain the bonding. The stated bond-dissociation values compare complete dissociation processes, so their magnitudes should not be derived solely from one overlap integral or from the two atoms' electronegativity difference.
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