Im surprised none of the nextdifficulty.com numbers match the real time one. And its the simplest method how to predict difficulty so I dunno why they dont use it (actually you need some blocks found to even compute it, but after first day, the real time prediction is usefull).
Actually the leftmost conservative number is 'realtime' based on last 1500 blocks mined (regardless of to which diff period they belong to) so it tries to grab some recent trend. I saw no reason adding shorter term calulations because they just add confusion to people who don't understand variance and other statistical stuff behind difficulty.
After block 1500 it becomes the realtime diff estimate as you would call it because it starts to include all mined blocks in current difficulty
