Boring hole was easier with 4 jaw chuck as opposed to mill, why?

[gheumann]

Depends on your tooling.
In the lathe you can make a flat bottomed blind hole with ease.

In the lathe, a boring head will give you greatest control on diameter, but you need a very expensive boring/facing head to get the flat bottom.
Alternatively, you could put the part on a rotary table and use an endmill to cut the flat bottom.

When I want a flat bottom hole on the lathe, I finish with a boring bar with indexable inserts (triangle shape). I set the tool angle so that there is relief when cutting the bottom of the hole. Cut to desired diameter, then move the tool toward the center. Precise diameter, flat bottom. Why do I need a special "boring/facing head"? (You may be talking about more accuracy than I need but I can reliably do this within .001")