I don't know if I am just imagining this, but I am pretty sure that the simulated gacha on this site hasn't actually been updated to reflect the proper pool of servants and CEs available when summoning for specific banners. An example I can think of is that Karna or Arjuna should be in the summoning pool for any banner, but they never appear, and there are quite a few newer CEs that should also appear in general rolls but do not outside of the banner they appeared in.
So I guess this is more a question for Gamepress than for the community, but I suggest you guys update the summoning pool to better reflect the current availability in NA, just for the sake of trying to be as genuine as possible with the simulation.
I also think this encompasses changing the rate-up proportions now that NA has actually published all the rates for every single item that can be claimed on a specific banner. We always knew that rated up SSRs were 0.7%, but I think Gamepress was working under the assumption that SRs were also proportionally 70% of the SR chance (3%), which we now know is not the case (it's 50%, or 1.5% rate for a rated up SR). I think you guys need to revisit these algorithms as well, now that it has all been made transparent. I only say this because it seems like it's closer to 70% than 50% for rated up SRs on the simulated rolls as averages, but maybe not, maybe you guys already have that one right? Either way, I just wanted to point out that the simulated gacha needs updating in certain areas, so if you guys get the chance, please work on that.