Body height 100% displaying vertical scrollbar

Question

Out of curiosity, considering the example below, why does having the margin on the #container div cause a vertical scrollbar to appear in the browser? The container is much

Answer 1

Based upon @BoltClock♦'s answer, I fixed it by zeroing the margin...
so

html,body, #st-full-pg {
   height: 100%;
   margin: 0;
}

works where id "st-full-pg" is assigned to a panel div (which further contained panel-heading and panel-body)