The amount of LISK shown at the ICO page is only a current estimation how many LISK you will receive. After the ICO we will distribute a fixed 85,000,000 LISK.

That means the more BTC get exchanged, the less LISK everyone get for 1 BTC. At the same time this means, 1 LISK is worth more. In the end you will receive the exact same amount of money you participated with, only the amount of tokens differs.