MongoDB: $inc generating some wrong value when incremented with negative decimal value

Question

While using $inc to decrement a value by some fractional value,ganerated output differs with expected output.

Steps to reproduce:
1.Insert

db.test.insert({"qty":4.464}

    )

2.check results

db.test.find()

3.increment the value with negative fractional value

db.test.update({},{$inc:{"qty":-0.608}})

4.check the output

db.test.find()

Actual Output: "qty" : 3.8560000000000003
Expected Output: "qty" : 3.856