[ubuntu] Why did Ubuntu 15.10 install itself onto an Ext2 logical partition?

I recently bought a SSD drive. I formated it into a swap partition and an Ext4 partition and then installed Ubuntu 15.10 from a CD (and inteet). I ticked the box LVM during the installation as I couldn't see any disadvantages to using LVM. The install went fine and the SSD is working even better than I expected.

Having done some reading up on using SSD's I decided to implement Autotrim but I have discovered that my Ubuntu instalation is on a Ext2 virtual partition (/dev/sda2) inside an lvm2 pv partition (/dev/sda1). My swap partition has been removed and in its place a /boot partition on Ext2 has been created. The /boot partition is not inside the lvm partition.

I am not overly conceed as I can easile run

sudo fstrim /