My guess is that your posting spec for 1031+ has the cash item defined as payment currency and the FX Security item defined as position currency. In this case the position currency is EUR. SAP therefore creates two documents, a USD document for the cash and a EUR document for the FX Security. SAP automatically creates the offsetting lines in each currency using the clearing account.
In my system, a similar transaction posts as below:
Hopefully this helps,
Mark