Justice In Oregon?

From the Associated Press, HERE is the complete article, dated January 3, 2016.

This has me puzzled:

Dwight Hammond, 73, and Steven Hammond, 46, said they lit the fires in 2001 and 2006 to reduce the growth of invasive plants and protect their property from wildfires.

The two were convicted of the arsons three years ago and served time - the father three months, the son one year. But a judge ruled their terms were too short under federal law and ordered them back to prison for about four years each.

Does this kind of re-sentencing happen often?

Is there a precedence for such re-sentencing?

Help me to understand this.