> Why would a business require that a high impact incident to have an urgency of low

A server down (a high impact incident) in a lab environment may not need to be resolved immediately, therefore may have an urgency of low.

> Why would a business require that a low impact incident to have an urgency of high.

If the CEO can't view their email, this would be a low impact incident (affecting 1 single user) with a high urgency (as the CEO must at all times have access to their email).