I ask because I recently ordered from a very reputable vendor, one who has almost 100% excellent ratings and everyone compliments his shipping punctuality. This is the reason I ordered from that person - the quick shipping.
In his vendor page on Agora he says that product always ships within 24h. It's been longer than that, and despite having the order 'confirmed' within 12h, it was never marked 'shipped' within his specified time frame.
Do some people not mark it 'shipped' immediately for some reason?
Yes some vendors don't mark shipped immediately.
One possible reason is that the market website (Agora perhaps?) is slow. It can take hours, especially during peak times, to mark all orders shipped, which isn't nearly as important as the actual shipping.
Some vendors also don't mark shipped immediately because their shipping setup is a different place from their computer setup.
Also possible that your vendor is just behind, or took the day off. I wouldn't sweat it.