Difference between varying a matrix or a variable over time

Hi everyone.

Yesterday i asked something about how to vary a variable over time in a specific form. Today i have another question, the subject is a bit similar, but it's actually something that is bugging my mind because of the results.

My question is: I have a RotationMatrix and i want to interpolate the rotations over time (from instant 0 to instant 1). I've done it with two approaches:

-> For the first case, i extracted the angle from the rotation matrix with the atan2, and then interpolated the value: angle(t) = angle*t. And it works normally.

-> For the second case, i vary the Rotation Matrix directly over time and then extracted the resulting angle with atan2. The way i vary the matrix was: rotationMatrix(t)= (1-t)*I + t*rotationMatrix.

And it works too, but like it's shown in the above image, the interpolation values are completely different. Wasn't it suppose that both approaches give same results? How can I formulate Case 1 to give me the same results of Case 2?

Thanks :)