Yes, it is a scoped point but DanielĀ“s hint was absolutely right and showed me the problem:

the force is applied to the point in one step and then withdrawn in the next one (step1: 0 to 500N, then step2: 500 to 0N) but I did the same with the point displacement without realizing that being a non-linear situation the system remains in a deformed state before the force has come back to 0 in step2 while with the "returning" enforced displacement, this one obligues the point to go back to its original position, thus forcing the structure beyond its "natural" final position. In fact, results after step1 were coincidents in both approaches, only in step2 the results diverged. With this explanation, now everything seems to fit in so I feel confident this is what happened. Would you also agree with it?

Thanks a lot to both of you for your help.