The study of beam uniformity and divergence in multi-beam-group-based negative ion (H) sources is crucial for the development and optimization of large-size NBI (neutral beam injection) systems with large extraction areas and multiple beam groups. One such RF-based ion source, i.e. the ROBIN ion source(single driver), comprising two vertically stacked beam groups (top and bottom), is considered to explore beam homogeneity and its correlation with the source plasma. In the ROBIN source, a vertical asymmetry in plasma density and temperature arises due to the presence of a transverse magnetic filter field based on permanent magnets near the extraction region. A similar but complementary asymmetry in the top-bottom beam group parameters is observed in ROBIN using a transversely placed (1.9 m along beamline) visible-range CCD camera. The role of extraction potential (i.e. directly related to the beam and plasma) in the context of meniscus penetration depth (: depth of the extraction field into the plasma) in this observed asymmetry is discussed in this work.