Why somebody put that big wall at 99.5 ?
Imagine you want to get rid of 1'000 coins at the best price possible. You know that when prices move up, the movments will likely stop at a wall. So your best bet is to set the limit to just below the highest wall you think won't be crossed. So in this case, the seller believes that Bitcoin won't raise above 100 USD anytime soon, but also thinks that it will touch 100 USD. In that case, you can be pretty sure to sell everything at 99.5 while still getting a good price.
As a corollary: if someone sells at 100.5 USD, the intention is likely a manipulation: here, the seller does not want to actually sell, but rather scare potential buyers by making the sell wall appear big -- that can pay off when doing large OTC transactions that take MtGox as reference.
ok, thank you for explaning
