The experimentally observed in/out detachment asymmetry in KSTAR L-mode plasmas with deuterium (D) fueling and carbon walls has been investigated with the SOLPS-ITER code to understand its mechanism and identify important atomic processes in the divertor region. The simulations show that the geometrical combination of a vertical, inner target with a short poloidal connection from the X-point to the target and a much longer outer divertor leg on an inclined target lead to neutral accumulation towards the outer target, driving the outer target detachment at lower upstream density than is required for the inner target. This is consistent with available Langmuir probe measurements at both target plates, although the inner target profile is poorly resolved in these plasmas and further experiments with corroborating diagnostics are required to confirm this finding. The pressure and power loss factors defined in the two-point model (Stangeby 2018 Plasma Phys. Control. Fusion60 4; Kotov and Reiter 2009 Plasma Phys. Control. Fusion51 115002; Stangeby and Sang 2017 Nucl. Fusion57 056007; Moulton et al 2017 Plasma Phys. Control. Fusion59 6) of the divertor scrape-off layer (SOL) and the sources contributing to the loss factors are calculated through post-processing of the SOLPS-ITER results. The momentum losses are mainly driven by plasma-neutral interaction and the power losses by plasma-neutral interaction and carbon radiation. The presence of carbon impurities in the simulation enhances the pressure and power dissipation compared to the pure D case. Carbon radiation is a strong power loss channel which cools the plasma, but its effect on the pressure balance is indirect. Reduction of the electron temperature indirectly increases the momentum loss and increasing the volumetric reaction rates which are responsible for the loss of momentum. As a result, the addition of carbon saturates the momentum and power losses in the flux tube at lower upstream densities, reducing the roll-over threshold of the upstream density. The relative strengths of the various mechanisms contributing to momentum and power loss depend on the radial distance of the SOL flux tubes from the separatrix (near/far SOL) and the target (inner/outer target). This is related to the strong D2 molecule accumulation near the outer strike point, which makes the deuterium gas density at the outer target 2–10 times higher than that at the inner target. A large portion of the recycled neutral particles from both targets reach and accumulate in the outer SOL, which is predominantly attributed to the target inclination and gap structure between the central and outboard divertors and hence to the impact of geometry. The accumulated neutrals enhance the reactions involving D2, which causes momentum and power loss.