Yesterday I put in an order to short SPSN at 6.83. Today I was filled at 7.19 which was much better than I thought it should have been since even the open was 7.01. I emailed my broker and he told me they use the middle price between bid/ask for papertrading accounts. I asked why not use the real bid ask and he said this: Is the part about getting filled in-between the bid/ask in liquid securities true? I've never heard of that and it doesn't make sense to me actually. I'm suspicious that this paper trading platform is way inaccurate since I think I "should" be negative on that SPSN trade (open was 7.01 which should have filled me, close was 7.15 and I was short. Instead entering at 7.19 makes me positive on that trade)
Some simulators are better than others. A simulator that always fills you at the midpoint of the bid/ask spread is not one of the better ones.
The broker wants to build up your confidence so that you trade with real money and the broker can collect commissions.