Photos grouped by multiple years

Why multiple years being grouped in the Years view in the Photos?

In my Photos in Mac, when click “Years”, then I can see my photos by each one year - “2011", “2012" and “2013".

However next group is by “2014-2018” which is multiple years.

I have checked each photo have correct timestamp. The number of photos are follows:

2014 around 250 photos

2015 2 photos

2016 Zero photo

2017 14 photos

2018 1 photo

I would appreciate if you could help to separate (group) by Year between 2014 and 2018.

MacBook Air 13-inch, mid 2011, Mac OS High Sierra Version 10.13.2.

Thanks for your help.

Best regards,

Taeko