They don't know what the "equals" sign means.
Seriously. Not a joke. American students are more likely than students in the control groups to solve 4+3+2=( )+2 with a 9 rather than a 7. That is, they misunderstand the entire question, and get the wrong answer.
The Science Daily story gives more detail, but we are fascinated by the possible ramifications of a nation in which many people cannot correctly interpret a single, unambiguous symbol. What other symbols do people not understand? A stop sign? A yellow light? And if they can't interpret these, what about more densely encoded but only marginally more ambiguous forms of symbolic speech -- a politician's obfuscation, for example? Or the teachings of a church?