UP Studio put highest priority on print dimensional accuracy and success rate. So if it detects empty space between platform surface and model bottom, even when selected no support and no raft, its algorithm will still add extra layers to fill up the space.
If there were no extra layers, the nozzle will likely print on air and result in detachment of the print, which may then result in snowballing of material onto the extruder nozzle. Even the print manage to finished, its dimensional accuracy will likely be compromised.
Following figure is an example; the left triangular shape is laid flat on the platform surface; the right triangular shape has a 0.5 degree tilt on X axis. Although 2 shapes appear to be the same but the right one actually has an unapparent gap between the platform and bottom. As a result an extra layer is added to fill up the space.