I’m not a very smart man. I’m trying to count how many years I’ve been working at my new job. I started in May 2011.
If I count the years separately, I get that I’ve worked 4 years – 2011 (year 1), 2012 (year 2), 2013 (year 3), 2014 (year 4).
But if I count the years as intervals, I get that I’ve worked 3 years 2011 to 2012 (year 1), 2012 to 2013 (year 2), 2013 to 2014 (year 3).
Have I worked 4 years or 3 years?
For a short analogy: I was born in 1986, so I am 27 now. But I have lived in four different decades (the 80s, the 90s, the 00s and the 10s). This is not the same as forty years.
In the same way, you worked in four different years, but not for four years.