Why UOM (Unit Of Measure) is selected as each when PSA shows other values?

As per the design, UOM always defaults to 'Each' and determines the label displayed under the price itself whereas the Billing Period applies a tag to the line item.

As per the design, UOM always defaults to 'Each' and determines the label displayed under the price itself, whereas the Billing Period applies a tag to the line item. 

For example, in per-user pricing, a single user is 'each' but it's billed monthly.
In the above instance, the unit of measure for a user isn't 'per month user', it's 'each user'.