There are two separate things:
the filesystem, a data structure that provides a way to store distinct named files, and
the block device (disk, partition, LVM volume) on inside of which the filesystem lies
resize2fs resizes the filesystem, i.e. it modifies the data structures there to make use of new space, or to fit them in a smaller space. It doesn't affect the size of the underlying device.
lvresize resizes an LVM volume, but it doesn't care at all what lies within it.
So, to reduce a volume, you have to first reduce the filesystem to a new size (
resize2fs), and after that you can resize the volume to the new size (
lvresize). Doing it the other way would trash the filesystem when the device was resized.
But to increase the size of a volume, you first resize the volume, and then the filesystem. Doing it the other way, you couldn't make the filesystem larger since there was no new space for it to use (yet).