TimeStamp clarification

I just now read the doc in http://www.unix.com/tips-tutorials/35535-understanding-unix-timekeeping.html

Need clarification that half of the files in any directory is shown in below pattern in which time stamp pattern is highlighted

And half of the files are shown in different pattern with time stamp pattern highlighted

Can you please highlight the reason?
How is this achieved?

The reason is explained in man ls
The -l option show dates for files older than six months or with dates in the future with a date format that the year in it - otherwise no year.

Thanks a lot