Why do we lie?

I was on my way for a client meeting with a colleague, given it would be rush hour we thought of travelling local by first class. On my way to station, I was trying to recollect likely ticket cost I paid last time for journey. I thought it would cost around 240 per ticket and we were 2 so it would be 480. I removed 500 note from purse and walked towards ticket window. I handed money and said “first class, 2 churchgate return”, person nodded and started punching on computer and took money. He handed me 20 rupees and ticket. I took it and was moving away from window towards station he called “hey wait”. I came back to his window and he handed me cash worth Rs260/-. I was shocked, I was wrong by miles, each ticketed cost Rs120/-. Ticket cashier had opportunity to take that extra cash home, but he didn’t. Makes me wonder, what makes one lie?