How come the grey market prices changes every day?

Grey market premiums moves like listed stock prices. Ultimately it's based on demand and supply. More buyers then sellers will pull up the grey market premiums, while more sellers then buyers could pull it down.

Many things change when a company announces an IPO to the IPO listing day. Thus the grey market premium keeps changing every day, every hour.

Short selling is also done in grey market if investors think that stock price will go down after listing.

Note: As no regulatory body involve in grey market deals and no circuit filters in place, grey market premiums may go up or down drastically in case of any positive or negative news flow.