How does Fortran handle optional arguments and strings?

Question

I have the following code:

program test
    implicit none
    call test_routine("TEST")
    call test_routine("TEST",.true.)
end program

subroutine test_routine(var1,var2)
    implicit none
    character(*)      :: var1
    logical, optional :: var2

    write(*,*) len_trim(var1)
    write(*,*) var1
end 

The output looks like this:

           0

           4
 TEST

Why does this occur? I would expect the function to behave the same whether the var2 argument is present or not.