The Geneaology (D3)

Tom and Tim, time tested twice troublesome twins, entered the kitchen at ten o'clock on a Tuesday evening. "Mom, mom, look what I found" said Tom, waving a sheet of faded paper.

"No, I found it." said Tim.

"What is it, Tom?" Mom asks.

"I don't know, mom, but it talks about Genies."

"really?" she replied as she took the paper from him. It was a copy of the family geneaology she had been looking for so she could do some research. "where was it?"

"It was in that Bible on the mantle." said Tim, "Between page 588 and 589."

"No," said Tom, "it was between pages 1201 and 1202!"

Mom gave Tom a dirty look, and said to Tim, "thanks for finding this sweetie." She looks at Tom. "Why did you lie to me, Tom?"

How did Mom know that Tom was lying?