The Earth's rotation speed can be influenced by many things and can both be sped up and slowed down. For example, the Sumatran earthquake in 2004 shortened the rotation period by 3 milliseconds. In theory negative leap seconds could be added but none have yet.
This gives rise to problems for computers since they are irregular and unpredictable, and can't be made part of algorithms. In extent, to properly calculate a precise time difference (actual elapsed time) over several years, you actually need to consult a table of added, historic leap seconds and take these into account too...
Simple. What's happening is that in the past we were measuring seconds relative to the duration of a day, but since then we've switched to atomic clocks.
So the day is not 24 hours or 86400 seconds, but rather 86400.002 seconds on average. That's about 2 milliseconds or more of deviation per day and for a whole year that's about 0.7 or 0.9 secs worth of deviation. Yet we still pretend that the day has precisely 24 hours, hence the need for leap seconds.
Given that since 40 years ago since leap seconds were adopted about 25 leap seconds have been scheduled, that sounds about right.
This gives rise to problems for computers since they are irregular and unpredictable, and can't be made part of algorithms. In extent, to properly calculate a precise time difference (actual elapsed time) over several years, you actually need to consult a table of added, historic leap seconds and take these into account too...