The hyperfine splittings Full-size image (<1 K) and Full-size image (<1 K) are analyzed in the framework of an effective lagrangian possessing chiral, heavy flavour and spin symmetries, explicitly broken by a complete set of first order terms. Among these terms, those responsible for the difference between the couplings Full-size image (<1 K) and Full-size image (<1 K) are evaluated in the QCD sum rules approach. Their contribution to Δd and to Δb appears to quantitatively balance previously estimated chiral effects, in nice agreement with the experimental data, solving a suspected puzzle for heavy quark theory.