[xubuntu] Need help understanding kernel version numbering

Either I can't read properly or I don't understand keel numbering or something is not happening correctly. Back in February, I installed Xubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (as stated in the README.diskdefines file). At that time, the output from uname -a was:
Linux ross-HP-EliteBook-6930p 3.19.0-49-generic #55~14.04.1-Ubuntu SMP Fri Jan 22 11:24:31 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux.

Presently, after several Software Updater updates, when I do uname -a, I get
Linux ross-HP-EliteBook-6930p 3.19.0-49-generic #55~14.04.1-Ubuntu SMP Fri Jan 22 11:24:31 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

and when I look in /boot I see (among other things):
-rw------- 1 root root 5833568 Mar 17 15:51 vmlinuz-3.13.0-85-generic
-rw------- 1 root root 6574032 Jan 22 07:22 vmlinuz-3.19.0-49-generic
-rw------- 1 root root 6575024 Feb 26 15:32 vmlinuz-3.19.0-51-generic
-rw------- 1 root root 6579344 Mar 11 05:10 vmlinuz-3.19.0-56-generic
-rw------- 1 root root 6591152 Mar 18 14:27 vmlinuz-3.19.0-58-generic

It seems to me that I should be operating under 3.19.0-58-generic and that the other keels should be “older” keels, candidates for removal. Am I correct and if so, why isn't the system 'updating' keels correctly? If I am not correct, where have I gone wrong in my thinking?
BTW, I do not have a separate /boot partition and / is only 22% used, so I don't think it is a case of there not being enough space for a an update to occur.