OK, lets try a hypothetical.
You're DMing. Your (4th level PCs) players are exploring a dungeon. They're dragging their heels, and have been having a reasonably easy time of it; HP are close to full, the Barbarian has 1 rage left, most Hit Dice have been used, plus a few 2nd level slots. The PCs have enough time remaining for a Short rest however. The BBEG isnt far off (a few rooms away), and the players are getting a little side tracked with table talk and deciding what to do next. You decide to throw a quick 'random' encounter (to focus their attention, and weaken them a little for the BBEG) is in order. The session is due to end in around an hour or two.
You had nothing planned, so you roll on a table provided in the module you're running. You roll an '89'. The players dont see the roll, and have no access to the chart even if they did.
The result tells you '1d6 Werewolves'. You roll a 6. Werewolves don't really make sense in the context of the adventure.
You know your PCs have no silver or magical weapons, and this encounter will almost certainly result in a TPK. You also note that on the same chart, the result of an '88' gives a much more manageable encounter for them, and also ties into the BBEG a lot better (it's the BBEG's henchman, obviously sent out to 'deal' with the PCs).
Do you:
A) Just throw the Werewolves at them because 'the dice say so'?
or
B) Alter the result to an 88, and use the more sensible encounter, that better serves the story and the outcome desired (and the reason you rolled in the first place) and that wont result in a crushing TPK?
Which is it?