Optical sectioning plays an important role for in-vivo or 3D optical microscopy. We have developed reflection-mode dynamic speckle-field interferometric microscopy (DSIM) to image targets with high spatial resolution, as well as high depth sectioning. In DSIM, single-shot, wide-field quantitative phase image can be obtained by using broadband, time-varying speckle-field in Linnik interferometer combined with off-axis holography. With broadband laser (center wavelength 900nm, bandwidth ~ 100nm), and high numerical aperture objective lens (1.0NA, water immersion), we could demonstrate up to ~600 nm depth-sectioning. However, there was no concrete theoretical model to describe the system. In this study, we developed a theoretical model to investigate the depth-sectioning effect of DSIM based on the theory of optical diffraction tomography with 1st order Born approximation. We systematically studied the dependence of the depth-sectioning, and image contrast in accordance with the bandwidth, and numerical aperture, respectively, and it shows a good agreement with experiment. We also studied the effect of the aberration, and dispersion induced by the target specimen for better understanding of the system dealing with thick targets. In addition, our model can be easily extended for general optical coherence microscopy, and will help for better designing of such optical systems that requires high depth resolution.