Can a character somehow gain more, or less, hit dice than they have character levels?

Question

I find the wording of regaining Hit Dice during a long rest very odd:

The character also regains spent Hit Dice, up to a number of dice equal to half of the character's total number of them (minimum of one die). For example, if a character has eight Hit Dice, he or she can regain four spent Hit Dice upon finishing a long rest.

Instead, it seems like the rules could simply state that a character regains spent Hit Dice up to half their level (minimum of 1).

Is there some corner cases where a character can have more or less Hit Dice than they have character levels?

Answer 1

The existing answers cover future-proofing and edge cases (Aberrant Dragonmark). Those are good explanations, though there is a simpler one.

Monsters spend Hit Dice, too

A monster can take a short rest, spend Hit Dice to recover hit points, then later take a long rest to recover those Hit Dice. There's evidence of this in the rules for a Beast Master's companion:

Like any creature, it can spend Hit Dice during a short rest to regain hit points.

By referring to Hit Dice rather than level, the general rule for regaining spent Hit Dice easily applies to both PCs and monsters, even though the latter (typically) has no level.

Answer 2

You can only regain spent hit dice.

There currently exists no features that add hit dice other than spent hit dice. Since you cannot spend more hit dice than you can have, you can never regain more hit dice than you had.

So yes, your maximum hit dice is almost always equal to your character level (exceptions noted below).

The aberrant dragonmark feat can lose you one hit die.

An optional feature of Aberrant Dragonmark says:

If the character gains a boon, the DM chooses it or determines it randomly. The character also permanently loses one of their Hit Dice, and their hit point maximum is reduced by an amount equal to a roll of that die plus their Constitution modifier (minimum reduction of 1). This reduction can’t be reversed by any means.

Thanks to Kogarashi Kaito for pointing this out in comments.

The shapechange spell creates something of an exception.

Shapechange says:

You assume the hit points and Hit Dice of the new form.

So while under the effect of shapechange, you can have a different number of hit dice than your character level, though your statistics (which includes level) are replaced while in this form.

Answer 3

This is a bit of "future-proofing". At the time of writing, there was nothing in the game that could alter your number of hit dice, but this ensures the rules are already in place if something should be introduced later that does so.

The only example I can think of off the top of my head is in Eberron: Rising from the Last War as an optional rule attached to the Aberrant Dragonmark feat.

Option: Greater Aberrant Powers

At the DM's option, a character who has the Aberrant Dragonmark feat has a chance of manifesting greater power. Upon reaching 10th level, such a character has a 10 percent chance of gaining an epic boon [...] If the character gains a boon, the DM chooses it or determines it randomly. The character also permanently loses one of their Hit Dice, and their hit point maximum is reduced by an amount equal to a roll of that die plus their Constitution modifier (minimum reduction of 1). This reduction can't be reversed by any means.

A DM could also create a curse, poison, or disease that reduces a character's hit dice until it's removed, or invent a boon, blessing, or magic item that grants additional hit dice. In either case, this rule already covers how that would affect the character.