My impression is I should be getting my mhash regardless of how lucky my miners are?
Your impression is incorrect. There is variance in finding blocks. If the pool should have found 120 blocks in the 3 days you were mining but instead found 102 then you would receive 18/120 = 15% less revenue.
Also prop pools can be pool hopped. Pool hoppers steal your money (you earn less than expected value) and they earn more. Mine in hop proof pools (PPLNS, PPS, SMPPS, etc). Hoppers can reduce revenue of suckers like you by 20% or more.
You likely don't have 0 downtime. You can check. At 1.38 GH/s you should produce 1157 shares per hour. Check how long you have been mining to number of shares submitted.
Fees and stale reduce payment so even under optimal situations and without any variance you would never make 28 BTC. At best with 3.08% lost to fees and stales you would make 27.1 BTC.
Thanks, I'm still beginner when it comes to this stuff.
Now checking my deepbit it has increase to 0.72bitcoins/24h, ignoring variance with the difficulty and 1.38GH/s it should be around 0.93bitcoins/24h?
For deepbit past 24h the average solving is 0h 31m, how do I calculate if that puts me at 0.72bitcoins or if I should be getting more/less?
As for downtime I'm not sure, I have wery good internet it's 99.999% online