I'm aware of all this however you'd have to clarify with "70s era?" and so on (which is somewhat vague), but 1982 for example clearly wasn't "the 70s".
It's also about a person's perspective. One of the things I've noticed is that younger folks (that is, younger than I) almost always confuse the 80s with the 70s, etc. I mean, they'll see something from 1983 and always think that it literally is from, say, the mid-70s, which it clearly isn't . I'm old enough to accurately remember those days, and the distinctions between each year were fairly well pronounced, much more so than in later years. Once 1980 ended, that was pretty much the end of the 70s. I remember people remarking on it, at the time.
Last point: when American Graffiti came out in 1973, it was about events in 1962, only eleven years prior! There was this huge wave of nostalgia about the good old days ("where were you in '62?" was the line). They were looking back across a span of only eleven years like they were light years...can you imagine looking back from 2015 to 2004 with the same nostalgia? No.