The pool hash rate is changing all the time, but say it was 400TH/s when you plugged in (it's 422TH/s as I write this), and where in the round you connected affects the reward as well, but in terms of hashing power 10GH/s divided by 420TH/s times 100% = a relative increase of 0.0025% for the pool, so yeah, the reward bump wouldn't be a lot.
Do a lot of people turn off over the weekends? I always feel like the weekends have longer rounds during certain hours but slightly better rewards. I'm not plotting any data, mostly this is just very casual observation... I'm 24/7 with my equipment so I just assume most people are the same way.
I've also limited myself to just Slush's pool, it works well enough so I've had no motivation to bounce around or explore. Perhaps during the long blocks people drop out and come back a few days later. I'm pulling at straws, guess I'm just trying to find out what might make the pool hash rate decrease, assuming it ever does I'm aware it would only be temporary and the long-term trend is always building here...
Edit: just so I understand, generally if my rewards get lower and it's not from a difficulty jump is it safe to assume it is because the pool either has more members or more equipment plugged in?
Edit 2: okay, now my estimated reward is higher by the amount I was expecting, perhaps there was some lag in the system(?)