Basal ice-shelf melting critically impacts Antarctic ice sheet evolution. Our testing of two melting schemes showed starkly diverging projections despite near-identical initial states, especially for West Antarctica. By 2100, the predicted sea-level contributions differed by 57%. Initial setup changes hidden sub-ice properties (e.g., friction, temperature), modifying ice flow. Accurately representing melt and refining setup are thus essential to reduce projections uncertainty.