The AC frequency used to be very inaccurate, it was only through a lot of work and legislation that it was made accurate, specifically so that electronic device

The AC frequency used to be very inaccurate, it was only through a lot of work and legislation that it was made accurate, specifically so that electronic devices could use it to keep time. Nowadays the electric companies want more leeway in how they run their grids and would love to have the frequency be less accurate, trying to keep it accurate actually costs a lot of money, since now as load changes on the grid they have to make sure to spin up new capacity. As the load increases the frequency goes down, as load decreases the frequency goes up. It is a careful balancing act that is required for the power companies to keep it within spec. If it was allowed to drift more the power companies wouldn't be required to spin up more capacity that then goes unused.