Scalar implicatures involve inferring the use of a less informative term (e.g. some) to mean the negation of a more informative term (e.g. not all). A growing body of recent research on the derivation of scalar implicatures by adult second language (L2) learners shows that while they are successful in acquiring the knowledge of scalar implicatures, a property at the semantics--pragmatics interface, it remains controversial as to which mechanism, default or non-default, could account for L2 learners' derivation of scalar implicatures. The present study used an online self-paced reading task to address this issue by examining the role of the speaker's knowledge state in the interpretation of the existential quantifier some by Chinese-speaking learners of English in incremental sentence processing. Results showed that both L2 and native participants demonstrated comparable online sensitivity to the speaker's knowledge state. Critically, when the scalar implicature was computed in situations where the speaker was more likely to know whether the statement with the stronger alternative was true, it gave rise to measurable reading latency, indicative of increased processing costs. We conclude by arguing that our findings are compatible with the context-driven models within the Gricean tradition.