Even if they don't sell order flow, they may send your order to a single option exchange regardless of who has the best bid or ask, further reducing your chance of being able to buy at the best ask, much less at the best bid. Several times at Ameritrade, I tried to buy at the best ask and didn't get filled, because my price was below the ask at the particular exchange they sent the order to. (My order was displayed, and locked the market.) I called to complain once, and was told they don't guarantee that option orders will get filled at the best price.
If that's the case that they don't guarantee the best fill, seemingly a possible reason is because of a bias as to where to route the trades.