This was unexpected, and not documented at all in the SharePoint SDK.
When you create a UserMulti column, you must also specify Mult=”TRUE”, otherwise it will fall back to a standard (single select) User column type.
<Field ID=”{12312312-1234-1234-1234-123412341234}”
…
Group=”My Group”
Type=”UserMulti”
Mult=”TRUE”
…
THANKS GOD!!!!! I was having HEADACHE searching WHY my field was always always like a User type colum even with the “UserMulti”