The old standard, up until 1979, was based on the fact that 1800s equipment wasn't real accurate. K ratings were allowed to be +/- .5K, so 14K could be 13.5-14.5K
When technology improved, jewelers made sure sure you got the absolute minimum, pocketing the extra.
A new standard, called Plumb gold (usually indicated by 14KP) is only allowed .03K variance, almost 17 times less wiggle room. So 14KP must be 13.97-14.03K out of 24.