Why is the validity period of my visa longer than the number of days I have been granted a visa for (period of grace)?
When we issue a single-entry visa, we always add 15 extra days. This extra period of time is called "period of grace" and is meant to give you more flexibility, not a longer stay. It is important to remember that you cannot stay in the Schengen area for more than the number of days stated on the visa label.
For instance, if you have been issued a visa for 90 days, the to-from date will be 105 days. Remember that you only have permission to be in the Schengen area for up to 90 days during this period.