I didn't realize you could use ARCA when trading listed stocks. I have always sent my orders to the NYSE...
If I'm not mistaken IB is also a large equities trader and your competitor, and they know where everybodys' stops are, and you don't.
I think IB is not allowed to use this information. Anyway, the support for ARCA is better with other brokers because I remember IB has restrictions on the ARCA order types you can use.