The problem is for ALL Antminers unless you have installed the proper binaries from -ck and kano.  If you are using any other binaries (Bitmain's, some other forks for extranonce, etc) then you do not have the updated code that submits stale shares.  In other words, you are potentially throwing away possible block solutions.

As to your other question regarding luck... yes, I believe it is just poor luck.  We are experiencing normal swings in variance.  As I initially mentioned, S7s are certainly a potential influencing factor since there are no (to my knowledge, -ck/kano can correct me) updated cgminer binaries for it.  However, I don't know how many people currently mining on p2pool are using S7s, so I don't have any reference for how much of an impact it might have .