EPM - Forced Custom Extension?

Commercial EndPoint Manager: 2.11.0.1.20
FreePBX: 2.11.0.9
Cisco SPA303 fw: 1.01/1.02

When using extension mapping in EPM I select extension “001” which is of type SIP. Extension mapping automatically creates a custom extension of “1” and assigns it to the cisco phone. Does this make sense? Shouldn’t the extension remain 001 and not custom?