You need to understand a bit more about how Slush's pool works and you (and many other) get confused with the PPS (pay-per-share) concept. Slush's pool is not a PPS one.

The detailed description of how it works is on the pool's home page - look for a text in big yellow letters that says "How does bitcoin pool work?".

In essence each share gets a "score". The score is highest at the round end, and earlier shares get lower scores. The older the share the lower the score. And that progression is not linear but exponential, so score goes down to almost zero very quickly. From observations I think the cut-off is about an hour - for shares older than an hour you get absolutely no score.

For a slow miner like you that may result in a noticeable variance - sometimes your last share could be just seconds before the end (and will get a high score), sometimes it could be a minute before the end (and will get a lower score). On some very short rounds you may not even have time to submit a single share.