Yes, I agree with your point about context sensitivity, Faldage and Jackie. OTOH, "19th century" is completely unequivocal for the whole caboodle. Strange thing is, that though I happily identify 1820s or 1980s as decades, that doesn't seem an obvious pattern below the twenties: 1910s doesn't automatically make me include 1917! No particular logic seems to be at work - anyone else find thsi pattern?

edit:Afterthought - maybe it's simply because there is less lexical uniformity below ~20. We sometimes say nineteen-oh-eight, sometimes ~and-one (a space oddity), and the teens are notoriously irregular