Why is the exchange rate pass-through usually less than perfect (i.e., less than 100 percent)?

Question:

Why is the pass-through usually less than perfect (i.e., less than 100 percent)?
Exchange Rate
The value of one currency for the purpose of conversion to another. Exchange Rate means on any day, for purposes of determining the Dollar Equivalent of any currency other than Dollars, the rate at which such currency may be exchanged into Dollars...