Possible problem with free_symbols

If I would like to know free_symbols of an expression, I found:

using SymPy
@syms x y
(x+y)[:free_symbols]
# output:
# (::__XXxxXX__) (generic function with 1 method)

while

([1 0;0 1]*(x+y))[:free_symbols]
([x 0;0 y])[:free_symbols]
# output:
# PyObject {x, y}
# PyObject {x, y}

Questions:
1. How does the `free_symbols` work on non-array expressions different from arrays ?
2. Why it returns as `PyObject {x, y}` instead of `Array{SymPy.Sym, 1}` ?