I think I just didn't get it all the last time around. It was pretty prolific and low growing so I probably missed some.
The root will grow for a long way underground. The visible leaves can pop up anywhere along the root. What I have pop up in the yard now is usually from a root I missed.
Brush-b-gone does work better for the big stuff (root etc), you can just paint it on the root where cut and it really works great.
I need to post some pics tomorrow of some tremendous poison ivy growth at my place of employment. Looks like branches of the tree, but its not.
Seeds are not killed by Roundup or BrushBeGone, I believe. So you've got to get it when it sprouts early in the springtime to be effective all season.............