Casting array to IEnumerable<T>

Assume you have a basic Employee class as such:

class Employee
{
   public string Name;
   public int Years;
   public string Department;
}

Then (in a seperate class) I have the following code fragments (I think i understand all but the last):

I believe that the following code fragment works because the array initiliser creates an array of Employee objects which are the same type as the workforce variable being assigned to.

Employee[] workforceOne = new Employee[] {
   new Employee() { Name = "David", Years = 0, Department = "software" },
   new Employee() { Name = "Dexter", Years = 3, Department = "software" },
   new Employee() { Name = "Paul", Years = 4, Department = "software" } };

I then have the following code fragment. I believe this works because the array of Employee objects implicity is an implementation of the Array() class which implements IEnumerable. Therefore, I believe this is why the array can be assigned to IEnumerable?

IEnumerable workforceTwo = new Employee[] {
   new Employee() { Name = "David", Years = 0, Department = "software" },
   new Employee() { Name = "Dexter", Years = 3, Department = "software" },
   new Employee() { Name = "Paul", Years = 4, Department = "software" } };

Then I have this code fragment:

IEnumerable<Employee> workforceThree = new Employee[] {
   new Employee() { Name = "David", Years = 0, Department = "software" },
   new Employee() { Name = "Dexter", Years = 3, Department = "software" },
   new Employee() { Name = "Paul", Years = 4, Department = "software" } };

I am not sure why this code fragment works? IEnumerable<Employee> inherits from IEnumerable (and overrides (or overloads?) the GetEnumerator() method) but shouldn't i therefore need a cast for the above to work