Difference between swarf's single pass and trim from top

[ltomuta]

I need to swarf a slot and I'm trying to do so incrementally, in order not to load the small tool. Looks like I can do this with cutting mode "trim from top". But, unlike when I use single pass, the simulations seem to show that in this case the tool does not go as deep axially.

With single pass: 

With trim from top - ball end mill's tip does not "scratch" the model, only the side cuts.

As I cannot do a section analysis on the simulation, I cannot see if there is really material left there or is this just a simulation artefact. What would you say it happens here?