it is because of time dilation according to relativity
In the field of relativity in Physics, the difference in the occurrence of time measured by two different clocks is known as time dilation. The reason for this may be due to velocity or difference in gravitational potential.
In this field, there are relative time, and absolute time ( actual time)
the absolute time is numerical or calculated age of events while the relative time is only concerned with the sequence of events.
The relative time involve time dilation, which itself can be measured differently for any moving object than object that are not in motion.
Therefore, the actual time will be longer than the relativity time because of the time dilation.