The most common answer to this question is because non-linear effects (e.g. compression-only supports) are not active in load cases, but are in load combinations.
As a result, supports or connections that should release in tension/compression will still behave linearly in load case results, leading to seemingly incorrect forces.
Here's why
In FEM-Design non-linear behaviour is possible to consider for supports (e.g. uplift), connections and truss members (e.g. tension-only). The analyses where this is included are Load combinations, Imperfections and Stability.
Figure 1. Analysis types that include non-linear effects.
To activate non-linear effects, tick the “NLE” checkbox at Calculations > Analysis > Load combinations > Setup by load combinations.
Figure 2. How to activate non-linear effects.
Since these boundary condition non-linearities are not in effect for the results of Load cases, non-linear results must be viewed in Load combinations.
Figure 3. Non-linear support ineffective in Load cases (left), so tension is not released. In Load combinations (right), the non-linear behaviour is active and the results match the intended support condition.