It all evens out. I noticed this as well but you will see times where your hash is much higher as well.
Yes I identified 2 things :
- Because of the fixed difficulty, there is variance in the hashrate. Sometimes it will be lower but sometimes it will be higher and over 12 hours and more, it should be even as you said. We need a fixed difficulty because it makes the pool way more stable and efficient so we can handle more connected miners at the same time.
- A lot of miners have an unstable connection. If your connection is lost while you send shares, it will probably be lost or not accepted by the pool because of the delay.