Simple Case

Simple case

Robert was facing a paper when Kira suddenly hit his back. He jumped up and instantly screamed. Kira didn't care, she just wanted to know what her friend was doing. Although Robert is still surprised, she didn't care and asked, "What are you doing Rob?

He answered spontantly, "Count for your birthday..I think I am gonna give you a prize, kind of TNTs.."

"Really? I am so scare... huuuuuu."

She looked at a device Robert held, "What are you doing with that?."

"It has no bussiness with you, but, by the way, because you have surprised me, you have to help me. This item doesn't work well. I can't figure out the correct result of my arithmatics, compared to my own calculation."

It is a calculator. Kira said, "Mmm... let me see. Give me your note."

She looked at the Robert's writing, it showed: 123456789+987654321.
"What are you thinking with these reversed numbers? Anyway you can just use calculator.."

"THAT's what I was doing, ..but calculator show the different result.. I don;t know, it's just keep bother me..how could calculator wrong? Or my arithmatic is so bad?"

She compare the result, then pushed some numbers, and soon smiled to Robert. "I can't believe you could join this school. This is an easy problem." She showed display on calculator, but Robert is seem still not understand.

The display showed:
987654321 + 12345679 = 1000000000
But the result of Robert's note = 1100000000

"What's wrong with this? Hei Kira, you must tell me.. hei don't go..."

Kira run away.

The question is:
What's wrong with the calculator?

Preliminary

Hi. My name is Kira. It doesn't matter who am I and where am I, but let me take you to assist me. My favourite: case solving! Yes, let's play as a private detective. There are no crimes, no cops, no culprits. They are all only imaginations. If you have similar interesting cases, share with us. Ok.