Also I think the testnet blockchain is periodically reset, unlike the normal blockchain which retains its entries. This usually happens when a new major version of Bitcoin Core is released. Technically it isn't really a reset since a new blockchain for test coins is created and the old test coins continue to exist on the old blockchain.
From the Bitcoin wiki:
There have been three generations of testnet. Testnet2 was just the first testnet reset with a different genesis block, because people were starting to trade testnet coins for real money. Testnet3 is the current test network. It was introduced with the 0.7 release, introduced a third genesis block, a new rule to avoid the "difficulty was too high, is now too low, and transactions take too long to verify" problem, and contains blocks with edge-case transactions designed to test implementation compatibility.
