Why do I see a differnce in Time Spent per Visit(seconds) shown by metric and results manually calculated?

As you can see, TSpV shown by its metric is 124(123.53257388839309 in specific).

However, if you calculate TSpV:

2,353,172 / (36,066 - 14,438) = 109

it shows a certain amount of difference than shown by the metric TSpV(seconds).

Why is this happening?