I'd chalk it up to the timeless nature of Asgard; especially if you spend any significant time in the feasting halls of the honored dead themselves. That whole Fight/feast/(fill in the blank of your favorite activity here) all day and wake in the AM totally restored sort of thing would tend to make years or centuries potentially blur together into a single day memory wise IMO.

So, it may have been up to a century for Korra, but in Asami's subconcious time sort of folded itself more comptactly into something more like a day or two generating the impression "Ah, Korra's back from her latest round putting out of forest fires; all is now right with the world again" without how long its been truly sinking in until she stops to actually THINK about it.