One is a static estimate based on the file contents. After uploading it, OctoPrint will analyse the gcode file and try to estimate how long it will take.
Firmware settings such as your maximum movement speed and acceleration can drastically affect how long the print actually takes.
That's where the second number comes from, it constantly updates itself based on previous layers and other information OctoPrint derives from the print while it's in progress, and from a previous successful print of that same file (at least I think it does, I'm not 100% sure on that last one).
You could try using a more advanced estimator such as http://www.gcodeanalyser.com/ which may try to take into account movement speeds and acceleration.