so if p2pool.info is underreporting the pool hashrate, doesn't that mean it's over estimating our luck?
It isn't underreporting. It is using a smoother moving average that reacts less to temporary jumps and dips in hashrate.
When calculating luck, the full complement of individual 5 minute data points are used. The smoothing/averaging is only used on the UI/graph shown on the page to reduce the amount of data that has be loaded to the web browser and speed up the page load.