Question about interactive brokers platform

I'm testing their TWS platform currently on demo and there's something I can't wrap my mind around.

Say Current price is 1.8910/1.8915 for GBP/USD
Say I place a stop order to sell at 1.8885

It gets filled but at 1.8860.

I can't imagine a spread like this on an ECN? Is it that their demo platform is screwy? It's not like it's happened once either, it happens every time. 14 pips, 20 pips, etc.

Unless something's not right with my orders?

Their demo data is useless for such. To get valid data you must have a funded account and then get a paper trading account to test in.

Thanks, I knew it was kinda screwy but still wanted to ask.