This effect of context on human ratings is very similar to the one reported in BIBREF5 . They find that sentences rated as ill formed out of context are improved when they are presented in their document contexts. BIBREF5 suggest that adding context causes speakers to focus on broader semantic and pragmatic issues of discourse coherence, rather than simply judging syntactic well formedness (measured as naturalness) when a sentence is considered in isolation. 
 Question: What provisional explanation do the authors give for the impact of document context?
adding context causes speakers to focus on broader semantic and pragmatic issues of discourse coherence