As pointed out by a fellow colleague, the minus one is doing what exactly it is supposed to do, which is correct.

So, why is the need to subtract 1 from a ratio. Below is what I could think of.

Your expression, seems is trying to find a ratio of traffic volume for current year and past year, and to be specific its calculating the percentage CHANGE.

Lets pour in numbers...

Year -- Traffic

2015 -- 120

2014 -- 100

Now how would you find the % CHANGE ?

% CHANGE = [(VALUE(2015)-VALUE(2014))/VALUE(2014)] * 100

= [(VALUE(2015)/VALUE(2014)) - 1] * 100

= [(120/100)-1]*100

= [1.2 - 1] * 100

= 20%

Possibly that explains your MINUS 1.