I think it's time left in that file.
Exactly.
It's not a bug, but an arguably rather useless feature. The number refers to how long an individual file get be before the automatic track split (which is seamless, by the way).
If you go menu>rec setting>size, you can set the maximum file size. 2GB is the default. If you set it to 1GB, you'll see that the file size in minutes will change accordingly. The same thing happens when you change the "format" and "sample" paramenters.
If you want to know how much storage space is left on the card, you need to go menu>information>card, but you'll have to do the math yourself, unless I'm missing something.
On a related note, this might be helpful:
http://www.sounddevices.com/calculator/