Spectrometer-based or space encoded Fourier domain OCT is preferred because of its relatively simple design, fast measurement speed and good signal-to-noise ratio. Besides delivering structural information, it is often used to measure blood flow velocities. Commonly, the axial component of the velocity is calculated from the phase difference of consecutive A-Scans. While this result holds true for pure axial movement, a transversal component of the displacement will alter this simple relationship. We present a new model accounting for the changing intensity of the illuminating beam on the moving particles and explaining why the phase difference does not increase linearly with the velocity. Movements as small as 20 % of the beam diameter during the integration time of the line detector will alter the observed phase shifts noticeably. For small angles between transversal direction and direction of movement, the discrepancy between classically calculated and measured phase shift may be huge. At certain velocities and angles no correlation of the phase exists even so there is an OCT-signal. High velocities at small angles will result in a limit for the phase shift smaller than π. A safe region, where the deviations to the linear relationship between axial velocity and phase shift are small, is specified.