Testnet isn't the exact same as mainnet. With the fact that testnet is useless for most users (in their daily transactions), most people don't really mine with it. This results in the problem that the blocks would take forever to generate if someone stops mining. Hence, there is a rule that if a block isn't found within 20minutes, the difficulty is reset to 1.

Since the difficulty could only increase to a maximum percentage and the hashrate of the testnet is pretty high, blocks are generated very very fast for a prolonged number of blocks.