An action that lets you move a certain distance is separate from the pool of movement you have on your turn
There's a difference between an action or bonus action that grants you additional movement on a turn, and an action or bonus action that involves you moving as part of it.
The description of the Dash action says "you gain additional movement for the current turn" equal to your current speed. This references the general rules for movement and position, which state:
On your turn, you can move a distance up to your speed. You can use as
much or as little of your speed as you like on your turn, following
the rules here.
Your movement can include jumping, climbing, and swimming. These
different modes of movement can be combined with walking, or they can
constitute your entire move. However you're moving, you deduct the
distance of each part of your move from your speed until it is used up
or until you are done moving.
Other rules on movement, such as breaking up your move, also apply to this. In particular, the rule on moving between attacks states:
If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks. For example, a fighter who can make two attacks with the Extra Attack feature and who has a speed of 25 feet could move 10 feet, make an attack, move 15 feet, and then attack again.
Say you're 30 feet away from an enemy, and you have a speed of 30 feet. If something (e.g. the rogue's Cunning Action feature) lets you use a bonus action to Dash, you could move towards the enemy, use your action to attack them, Dash as a bonus action, and then use your remaining 30 feet of movement to move away from the enemy. (Moving away from the enemy might provoke an opportunity attack, but that's irrelevant to the current issue.)
In contrast, something like the gnoll's Rampage trait says:
When the gnoll reduces a creature to 0 hit points with a melee attack on its turn, the gnoll can take a bonus action to move up to half its speed and make a bite attack.
Notably, this doesn't reference the pool of movement that the gnoll has on its turn at all.
The gnoll has a speed of 30 feet, so it normally has 30 feet of movement for a turn. (If it takes the Dash action, it gains an additional 30 feet of movement for that turn, for a total of 60.) This pool of movement can be used freely on its turn, before or after its attacks (or between them, for the other kinds of gnolls that have Multiattack).
However, the Rampage trait doesn't relate to the gnoll's pool of movement on its turn at all. Consider how the Rampage trait would work if the gnoll doesn't have to move the full distance (i.e. half its speed) to reach another enemy that it can bite.
For instance, say enemy A is 30 feet away from the gnoll, and enemy B is 10 feet behind enemy A. The gnoll uses all of its movement to move 30 feet forward, then uses an action to attack and kill enemy A. The gnoll then uses its bonus action (thanks to the Rampage trait) to move just 10 feet forward and bite enemy B. After this occurs, the gnoll can't move any further – it already used its whole pool of movement on its turn to get up to enemy A.
If the gnoll doesn't move the full distance allowed by the Rampage action, the remaining amount doesn't carry over, because it's not added to its pool of movement available for the turn; it's just a separate thing that lets it move once (up to) a certain distance.