It’s not often accounting rules come up on company earnings calls. But that’s what happens when banks face the biggest change to their loan-loss declarations in decades.

A new set of rules upending how banks report losses on loans and set aside reserves to cover them is no ordinary bookkeeping change. Investors this quarter are peppering financial heavyweights like JP Morgan Chase CEO Jamie Dimon and Bank of America CEO Brian Moynihan with questions on how the shift will affect their bottom lines.

The answer? It’s all over the map. For many banks, the reason is simple: they just don’t...