Yes, it's because your particular encoder has 4 counts per detent and, as you say, sometimes it will "overshoot" the detent slightly causing the code register one count before it "springs back" into the detent position. The important thing is that the code DOES register it "springing back" so that it doesn't lose position.

These mechanical encoders exists with either 4 counts (one quadrature cycle) per detent or 1 count (1/4 quadrature cycle) per detent. Using 1x decoding with an encoder that has 1 count per detent means you'd have to turn it 4 clicks for each increment. However, with the type of encoder you have using 1x decoding might be better.