The printed rule says different
Do they?
DGM Page 4 under The Dungeon Master
"As a storyteller, the DM helps the other players visualize what's happening around them,
improvising when the adventurers do something or go somewhere unexpected."
And...
"And as a referee,
the DM interprets the rules and
decides when to abide by them and when to change them."
And...
"The D&D rules help you and the other players have a good time, but
the rules aren't in charge. You're the DM, and you are in charge of the game."
DMG page 5 under Master of Adventures
"The rules don't account for every possible situation that might arise during a typical D&D session."
DMG page 235 under Running the Game
"RULES ENABLE YOU AND YOUR PLAYERS TO HAVE fun at the table.
The rules serve you, not vice versa."
DMG Page 237 under The Middle Path
"
Remember that dice don't run your game-you do. Dice are like rules. They're tools to help keep the action moving. At any time, you can decide that a player's action is automatically successful. You can also grant the player advantage on any ability check, reducing the chance of a bad die roll foiling the character's plans.
By the same token, a bad plan or unfortunate circumstances
can transform the easiest task into an impossibility, or at least impose disadvantage."
DMG Page 263 under Dungeon Master's Workshop
"
AS THE DUNGEON MASTER, YOU AREN'T LIMITED by the rules in the Player's Handbook, the guidelines in this book, or the selection of monsters in the Monster Manual."
From what I can see the DM isn't obliged to follow what the backgrounds say if circumstances warrant a different ruling.