Strange behaviour of array concatenation and each_with_object

each_with_object.rb

def flatten_keys(hsh, keys)
  keys.each_with_object([]) { |k, result|
    result += hsh[k]
  }
end

def flatten_keys_each(hsh, keys)
  result = []
  keys.each { |k|
    result += hsh[k]
  }
  result
end

def flatten_keys_concat(hsh, keys)
  keys.each_with_object([]) { |k, result|
    result.concat hsh[k]
  }
end

hsh = {
  :a => [1, 2, 3],
  :b => [4, 5, 6],
  :c => [7, 8, 9]
}

p flatten_keys(hsh, [:a, :c])         #=> []
p flatten_keys_each(hsh, [:a, :c])    #=> [1, 2, 3, 7, 8, 9]
p flatten_keys_concat(hsh, [:a, :c])  #=> [1, 2, 3, 7, 8, 9]