ten blocks one every 10 seconds and one block once every 100 seconds gives you exactly the same amount of security if the overall hash rate is the same.
That might be true if the blocks were chained together, with each one depending on the previous one. But in P2Pool the share chain doesn't require this. The transactions appearing in one share may or may not appear in the following share in the sharechain and it just doesn't matter.
So it's possible you see your transaction in a share in the sharechain, and then see 10 more shares follow it, each of which not containing your transaction.
All that seeing your transaction in a share tells you is that a miner somewhere has accepted your transaction into his pool and is trying to mine it at the moment. It absolutely doesn't tell you that it will ever be successfully mined.
