I spoke to Tradeview over a demo problem. I also asked about this.

The guy said they had "three different types of account", and one was a "high interest" type. He said they "might take the other side of the trade only on those types of accounts".

I asked how Metatrader was managing straight-through processing, and he said they 'might release more information about that in future'.

That sounds reasonable to me, if its some proprietory thing.

[update]

Sorry, didn't see your message before I posted.

Interesting.

I am currently running both Alpari UK and Tradeview demos, with the same EA and the same settings. (I only started a few minutes ago). I will keep watch for data differences.