Callaway Golf’s materials and manufacturing advances have reduced the weight required to create the clubhead.
That allows their designers to ‘place’ weight to improve a particular performance characteristic of a club.
In the EPIC Irons, Tungsten is used to change the location of the CoG in each Iron. 

Almost all golfers struggle to get a high enough trajectory on
their long Irons. They rarely hit a #4 Iron any further than a
#5 Iron for instance. So the CoG is deliberately lowered
to create a higher launch in the longer clubs. 

Most golfers have no trouble getting shorter Irons airborne
on a good trajectory. In fact the last thing most need is a
higher ball flight. So in the shorter Irons the CoG is a
little higher to create more control over distance.   

When we conduct a fitting and swing assessment, we’ll also discuss with you where replacing an Iron with a Hybrid is the best decision.