Why Royal Navy class names can be ambiguous

Why not always name a British warship class after the first ship?

That is usual, but the identity of the first ship changes depending on whether order, launch, delivery, or commissioning is used.

The first ship of a class is not always self-evident because procurement and construction milestones do not necessarily occur in the same order. One vessel may be ordered first while another launches or commissions earlier. Different authorities can therefore select different lead ships without disagreeing about the class's composition. British class naming usually follows the first ship, but the meaning of first depends on the convention being used.