This is normal behavior and to be expected. The ideal placement of an SV100 is about half an inch from your mouth. They are designed to be right on top of. When working with SM58s (basically the professional big brother of the SV100), my recommendation to people who are unfamiliar with the mic is to hold it with their fingers wrapped just below the grill, stick out their thumb and touch their chin with their thumb. That's about the ideal placement.
Additionally, uncompressed, the human voice can be pretty dynamic depending on how you talk. If you move around at all, you will also change where you are in the pickup pattern of the mic, which will result in further level changes. -18 isn't a horrible level, it could be better by getting closer or turning up the gain, but it's more important to make sure you don't clip (have parts of your recording that exceed the maximum signal dB that the gear can handle).