googlebot1: Nope. The ideal 50:50 state can only be reached with an infinite number of samples. The actual occurences converge to the statistical value of "50% chance" on a coin toss. However, at any point before infinity (and I dare you try to reach infinity 
) there's also always the possibility (however small - or not) that there's a deviation from the ideal statistical ratio.
How else would a pool be able to say "oh, we're +20% on luck over the last 24 hours" or "-2% (= unluckier) during the last difficulty"? Although all events are unrelated and have the same probability, the real occurences will be much closer to average the bigger the sample gets. So although the theoretical average is 50/50, the actual average might be completely different. Think 10-times heads in a row. It's unlikely, but if your sample happens to be those 10 events, your average is not 50/50.
Real world example: My miner was offline for a couple of hours this night. Those were quite "lucky" hours, meaning the occurences of blocks were more than expected if you just use "ah, yeah, current difficulty - you should come up with a block per Gigahash every 16.5 hours". Now, being reconnected, the "luck" is below average so it would have been - in hindsight - advantageous to mine during the hours I was offline and have it offline now (although 24/7 operation would of course be superior to both).
tl;dr: So, yeah, the average is always the average but that does not mean you (or a pool) cannot perform above or below average. That's exactly why you got your block.
) there's also always the possibility (however small - or not) that there's a deviation from the ideal statistical ratio.How else would a pool be able to say "oh, we're +20% on luck over the last 24 hours" or "-2% (= unluckier) during the last difficulty"? Although all events are unrelated and have the same probability, the real occurences will be much closer to average the bigger the sample gets. So although the theoretical average is 50/50, the actual average might be completely different. Think 10-times heads in a row. It's unlikely, but if your sample happens to be those 10 events, your average is not 50/50.
Real world example: My miner was offline for a couple of hours this night. Those were quite "lucky" hours, meaning the occurences of blocks were more than expected if you just use "ah, yeah, current difficulty - you should come up with a block per Gigahash every 16.5 hours". Now, being reconnected, the "luck" is below average so it would have been - in hindsight - advantageous to mine during the hours I was offline and have it offline now (although 24/7 operation would of course be superior to both).
tl;dr: So, yeah, the average is always the average but that does not mean you (or a pool) cannot perform above or below average. That's exactly why you got your block.