Answer:
The automatic increase of the reinforcement for the SLS is based on the assumption of a lever arm, as it is also used for the ULS design. However, increasing the lever arm may change it. This occurs especially if the required reinforcement is applied with a different diameter than for the ULS, or is placed in a different position.
At the end of the calculation, however, the add-on module performs another design with the automatically designed reinforcement. In this design, the actual position of the reinforcement is considered. This can lead to the design ratio being exceeded, usually by a small amount.
Since the ratio is set to one decimal place according to the default setting, the overshoot may appear more dramatic than it is. In the add-on module, you can also set two decimal places under "Settings" → "Units and Decimal Places..."