You're missing the point. Bitcoin's target of 10 minutes per block is not a definite interval (where blocks can only be found precisely within 10 minutes of each other) but a probabilistic one (where blocks are to be found within 10 minutes on average). As with all things probability-related, there are always outliers. In Bitcoin's case, such outliers (i.e., blocks that have been found within seconds of each other) have been rather common. The decision to limit mainnet P2Pool's share size limit to an arbitrary value that's lower than what the Bitcoin network demands is a decision to completely disregard these (common) outliers. You therefore deliberately harm yourselves (intentionally limiting your earnings from transaction fees) and the Bitcoin network (intentionally being a bottleneck in Bitcoin's transaction confirmation process) by disregarding the very real and statistically significant probability of blocks being found in less than five minutes.

In other words, you are deliberately choosing to not take into account the inherent element of chance in the mining of each and every Bitcoin block. There is absolutely no guarantee that the next P2Pool block would be found only after five minutes from the previous block. There is always a statistically significant chance (albeit a small one) that the next mainnet P2Pool block would be found in less than five minutes after the previous block, due to how Bitcoin mining inherently relies on chance. Again, you not only deliberately limit your potential earnings within the first five minutes after each new block, but also deliberately become a bottleneck in the Bitcoin network's transaction confirmation process within those five minutes, acting against your own interests and the interests of the Bitcoin network.