Justice Elena Kagan, writing for the Court, treats this as a pretty easy question to answer:
As just noted, the elements of Mathis's crime of conviction (Iowa burglary) cover a greater swath of conduct than the elements of the relevant ACCA offense (generic burglary). . Under our precedents, that undisputed disparity resolves this case. We have often held, and in no uncertain terms, that a state crime cannot qualify as an ACCA predicate if its elements are broader than those of a listed generic offense. How a given defendant actually perpetrated the crime—what we have referred to as the “underlying brute facts or means” of commission, —makes no difference; even if his conduct fits within the generic offense, the mismatch of elements saves the defendant from an ACCA sentence. Those longstanding principles, and the reasoning that underlies them, apply regardless of whether a statute omits or instead specifies alternative possible means of commission. The itemized construction gives a sentencing court no special warrant to explore the facts of an offense, rather than to determine the crime's elements and compare them with the generic definition (internal citations omitted).Today's decision just reaffirms that it's the elements of the statute that matter for ACCA purposes, not the defendant's actual conduct. If a state's criminal code defines an offense more broadly than the "generic" offense under federal law, than it cannot be a predicate offense for an ACCA enhancement, even if the defendant's specific conduct falls within the narrower elements of the federally defined crime.