Looking at an eye pattern, the bit rate is equal to 1/"eye" (one crossing point to the next crossing point). However, the frequency measurement equals 1/period (from one rising edge to the next rising edge). This is equal to two bits. The result is the frequency measurement, in this application, is equal to 1/2 the bit rate. To verify this, turn on the annotations for the frequency measurement. Then turn on the cursors and place them across one eye pattern at the crossing points. The annotations will show where the frequency measurement was made; showing that from one rising edge to the next covered two bits. The cursor readout, 1/delta T will verify the frequency of one bit. This setup highlights why the frequency measurement will read one half of the bit rate.