I guess the probable reason is:
output of activation function = activation function (i.e. sigmoid/tanh) (matrix addition (w(i) * i(t) + b(i), w(h) * h(t-1) + b(h)))
which is different from:
output of activation function = activation function (i.e. sigmoid/tanh) (w(i + t-1) * matrix concatenation (i(t), h(t-1)) + b(h))) [as referred by the formulas from colah’s blog 3