It depends upon which way you are traveling from the decimal point. When you are going to smaller and smaller numbers, significant figures are determined by weighing and dispensing devices and sometimes instruments are limited in the numbers used in their read-out displays. When numbers start to get large, significant figures are limited because being very specific really has no meaning in relationship to the action level or reporting limit (for example: 1243 mg/kg would be reported as 1240 mg/kg when the reporting limit is 10 mg/kg, because the result is so large there is no reason to report every number possible from a read out).

For a more scientific explanation, this link has a great explanation using a graduated cylinder « scale »