why the second log files on /var/log are not compressed

I agree with you, that should only provide an easier access to recent files. Anyway, the actual behaviour is decided via logrotate 'delaycompress' directive, which says 'do not compress the file as you rotate, but compress it next time'.

The historical reason can be found in the logrotate manual:

Postpone  compression of the previous log file to the next rotation 
cycle.  This has only effect when used in combination  with compress. 
It can be used when some program can not be told to close its logfile and 
thus might continue writing to the  previous log file for some time.

Yes, this is done so that more recent log files are easier to access. The logrotate option delaycompress controls this. From the logrotate manpage:

delaycompress

Postpone compression of the previous log file to the next rotation cycle.  
This only has effect when used in combination with
compress.  It can be  used  when  some  program
cannot be told to close its logfile and thus might 
continue writing to the previous log file for some time.