Why would 2 aircraft fitted with the P&W PT6 Engine have different top speed?
The P&amp;W PT6 comes in many different varieties. The smallest PT have 500hp while the largest have 1700hp.  It is not the "same engine" as you state in your question.   

The standard Caravan has 675hp while the other aircraft you mention have 1,200hp. That alone can account for the major difference in performance. Fixed gear and struts also add to an increase in drag and a reduction in speed. 

The bottom line is they are different aircraft, with different engines, designed for different roles. There is no reason to expect similar speed or performance.