Why is it that if I count years from 2011 to 2014 as intervals I get 3 years, but if I count each year separately I get 4 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.

You started in May 2011.

By May 2012, you had worked 1 year.

By May 2013, you had worked 2 years.

In May 2014, you completed 3 years.

It won't be 4 years until May of 2015.

You worked 3 years between May 2011 and May 2014, and 2 extra months.

Note that in 2011 and 2014 you only work a part of the year, you should not count them as full years (as you do in the first count).