Quoted from barakandl:A ceramic cap can have 20% tolerance.
Many ceramics can be off by as much as 80%! Not long ago, 20% ceramics were the norm and 10% parts tended to cost more but the 10% parts have come down considerably. You can get ceramics as close as 5% but that tolerance tends to cost much more than 10%. At the time, Polyester caps were chosen due to it's tighter tolerance and tend to be more robust than a ceramic.