Since Lady Capulet and Tybalt are blood relatives, she would obviously, out of loyalty, defend his honour and be less believing about what Benvolio reports. Although he reports as accurately as he can, it is the terms that Benvolio uses when referring to Romeo's part in the fracas that makes Lady Capulet believe him even less.

Benvolio states:

"Romeo that spoke him fair, bade him bethink
How nice the quarrel was, and urged withal
Your high displeasure: all this uttered
With gentle breath, calm look, knees humbly bow'd,"

He is saying that Romeo was not aggressive at all and used gentle, conciliatory terms to calm Tybalt down, even telling him about the Prince's displeasure about fighting in the streets. Romeo was humble, calm and gentle in his manner. Tybalt was, however, "deaf to peace" and attacked Mercutio,  stabbing him when Romeo intervened:

"...underneath whose arm
An envious thrust from Tybalt hit the life
Of stout Mercutio."

Tybalt then flees but returns later. Romeo knows at this point that Mercutio has died and he has sworn revenge. He fights with Tybalt, killing him. Romeo then flees.

Lady Capulet obviously does not believe Benvolio's version of events, saying:

"He is a kinsman to the Montague;
Affection makes him false; he speaks not true:
Some twenty of them fought in this black strife,
And all those twenty could but kill one life."

She suggests that because Benvolio and Romeo are related he would, of course, lie. She states that the brawl had been a cowardly act and that Tybalt had been outnumbered twenty-to-one. It took twenty of them to kill Tybalt, suggesting Tybalt's bravery when he was grossly outnumbered. It is ironic that she takes this stance even though she had not even witnessed the encounter.

A further incentive for Lady Capulet's disbelief could be that, since she knows that the Prince and Tybalt are also related, that he would take her side, which the Prince does. He queries who should be punished for Tybalt's death and ignores Lord Montague's suggestion that:

"Not Romeo, prince, he was Mercutio's friend;
His fault concludes but what the law should end,
The life of Tybalt."

In other words, "a life for a life" - the score had been settled. The Prince commands that:

"And for that offence
Immediately we do exile him hence:"

and:

"I will be deaf to pleading and excuses;
Nor tears nor prayers shall purchase out abuses:
Therefore use none: let Romeo hence in haste,
Else, when he's found, that hour is his last."

He clearly displays favouritism.

In Act 3, scene 1 of the play, Benvolio gives a fair account of the fight, as you may have determined from reading lines 149-172.  (It is important to note that while Shakespeare might have considered having the Prince choose a more obviously impartial character to tell the story than Benvolio, he most likely did not wish to introduce a new character at this point, one that would be delivering crucial lines).

Lady Capulet's reasons for distrusting Benvolio's words are twofold.  First, she assumes that Benvolio's kinship with Romeo will automatically bias him against the truth: "Affection makes him false, he speaks not true" (174).

Second, Lady Capulet did not witness the duel, and had no doubt heard there were many men fighting in the general ruckus.  She apparently believes that Tybalt might have been attacked not only by Romeo, but by his friends: "Some twenty of them fought in this black strife,/And all those twenty could but kill one life" (175-176).