Late is the new black

I first heard the phrase 'fashionably late' in university. People kept on using it when they didn't turn up to lectures or other appointments on time. As far as I could tell, it meant that you could be late in a fashionable way, just as you could be tall in a fashionable way, or broad-shouldered in a fashionable way, or wear green in a fashionable way. But what the difference was between being fashionably late and late late, no-one could tell me. Presumably, if you wanted to turn up to an appointment fashionably late, and ended up there even later than that, you could be fashionably late at being fashionably late. Did that just make you extra fashionably late, or just late?

Also, you have to wonder, if people could be fashionably late for appointments, could trains be fashionably late? Could buses? Could kids stay up after bedtime, claiming 'fashionable lateness'? Why not 'fashionably punctual', or 'fashionably early' for appointments instead? (It somehow seems more possible to be punctual at being fashionably late than being fashionably late at being punctual. Though I could be wrong.)

What happened to this phrase, 'fashionably late'. Is it fashionable, lately? Or is it just, well, late?