Iphone 4 Read Receipts accuracy?

Okay so I have an iPhone 4 with IMessage and I have read Receipts turned on. But I'd like to know how accurate read receipts are. What does it go based off of? How does it know you really read it? I do it a lot with my boyfriend and there are times hell say "you read that like 20 mins ago" even if I just picked up my phone and saw. How accurate is it?

Definitely not a tax question. There are forums that cover cell phones; try there.