The 145Ghash is an estimate of the current pool speed over the last 30 minutes, As you can see from the graph the speed fluctuates somewhat.

Reward isn't calculated from the estimated speeds, but in fact from valid shares

Your reward is calculated like this :

Total Shares = Valid Shares in last (Difficulty / 2) Shares
Your Shares  = Your Valid Shares in last (Difficulty / 2) Shares

Your reward = your % of your shares in total shares.

You cannot make a simple calculation like 145Ghash / 6.621Ghash to get your exact reward. The projected reward is the most accurate.

Hope that clears things up a bit.