First, 14 is the median amount of Chanseys you need to encounter to get a lucky egg. The word "average" can refer to the median but usually refers to the arithmetic mean, which is 20 Chanseys in this case.

Second, you can use compound eyes to raise the chance of encountering Pokemon with items. The chance of finding a lucky egg on a Chansey will go up to 7.5%, so there's a 50% chance of getting at least one lucky egg in 9 Chanseys.