Hiya!
What this means is that fairness to the monsters is also showing fairness to the players. When you fudge dice in favor of the players, you down play the power of monsters and lessen the accomplishment of your players.
When you play your monsters as they should be, not only are you fair to the monster's capacities bit you are also making your players earn their victory. For without risks, there are no reward worthy of the challenge.
Here is an example. When CoS got out, I saw a group kill Strahd in quite an easy way, no challenge no risks.
Similar story...but it involves Tomb of Horrors.
During the 80's and 90's, in the small city where I live (population at the time was about 12k, up to about 16k in the late 90's), there weren't many RPG'ers. There was no "hobby shop" for a good long stretch (only a toy store that carried a single section of about 4' x 4' of RPG/D&D stuff).
At any rate, I quickly developed the reputation of "a really good DM" (...and yes, my early/late teenage self was rather proud of that reputation...still am, tbh

). I guess some of the more...hmmm... "self confident" Players in the area were convinced that they were AMAZING players and that their PC's could take on any challenge that TSR could throw at them.
One group was so convinced of their greatness, and their "teenth-level PC's" that they kept bugging me to DM them. So I did. Tomb of Horrors. None had even heard of it, so in they went. TPK in about 15 minutes of play. Some died to poison, the rest...yeppers... Green Demon Face. "He just kinda disappears? Must be teleport! Everyone in!" POP! POP! POP! POP! POP! Of course, they were used to using Player Knowledge; after the first guy went POP!, everyone waited for me to describe what happened to him...where he was, what he saw, etc, so they could decide what they were up against in wherever they were 'teleported to'. I didn't do that. I sat their and smiled. "You guys aren't there, so I'll wait to you decide if you are going through or not".
They were...simultaneously shocked and elated. I told them not to worry, it was a "test adventure" and their PC's weren't really dead. ...and yes, in those days, if your PC died in some other DM's game/campaign world, word got around you died and your PC was DEAD. As in, no, you can't use him in your regular game, because he's dead. They were extatic because they actually felt like their choices mattered. They realized that the game is soooo much more fun when you don't "know" what is going to happen, based on the DM not wanting to let the PC's die. You know...risk/reward.
Anyway, that story got to another player with a TOTALLY Munchkinized Paladin (with +5 everything, Holy Avenger and a Huge Ancient Gold Dragon for a Paladin Mount...with all the trimmings...). "Pfft! No way would I die! Try me!".

Ok...have you ever played Tomb of Horrors?
(spoiler alert: Paladin dead in under 5 minutes; twice; first 2 minutes, he fell in pit and failed his poison save with a natural 1! LOL!...but I "let" his Gold Dragon 'raise him'. He came back and decided to Fly down the hall way. Poked his holy avenger into the Green Demon Face. POP! "No way am I loosing that sword!" ...so in he went. POP!
I see this same sort of 'mindset' in a lot of players nowadays. But that is a whole other discussion thread!
^_^
Paul L. Ming