If you object's overall size is not an exact multiple of the spacing & thickness [allowing for the last-most slice/gap], then this will occur.
It looks like the start/end gaps are 1/2 a gap ?
Also choosing to centralize the slice can exacerbate this result - try centralizing the gap ?
You can also tweak the start/end insets to adjust the sliced result.
Supplying the model would help us see what's what...