-(이)거든요 is basically used to explain a reason. Native Korean speakers use it variously in spoken Korean.
To explain a reason :
A : Why are you late? 왜 늦었어?
B : Sorry, it's because of the traffic jam. 미안해, 차가 좀 밀렸거든.

To say a fact :
A : The boy looks cool, what do you think? 쟤 괜찮은 애 같은데, 어때?
B : I and he were classmates at school, he's sorta sly. 걔 내 동창 친구거든, 좀 얍삽해.

To answer in defensive way :
A : Hey! You're late! 야! 늦었잖아!
B : No. It's 4:59 pm. 아니거든. 지금 4시 59분이야.
If B say it in questioning tone, maybe it will become kinda aggressive.

Let's see two examples to show when -(이)거든 become sarcastic.
A : I didn't finish my homework, you didn't finish yours, did you? 나 숙제 다 못 했어, 너도 안 했지, 맞지?
B : I did it. 했거든.

A : You're interested in me, right? 너 나한테 관심 있지?
B : No. 아니거든.

Hope this helps.