It takes very long until bitcoin-core has downloaded the whole block chain. So I decided to download the block chain files via a bittorrent published by this website: http://www.getbitcoinblockchain.com

After the download was finished I noticed that the md5 checksums of the files from the torrent were different then of the files from bitcoin-core.

Why could that be?

Are there any security related concerns of using a block chain from a 3rd party source?