Quote from user83248324:
Yes, slippage. When you trade a spread, someone takes the other side of the trade. That person is not going to give you 15-20 cents when the stock is unchanged.
IMHO TOS is a great broker. But, I believe they give you 'fills' in paper trading accounts that are unavailable in the real world.
Why do that? To give you confidence and get you to enter real orders. Just a guess.
Mark