When you try to generate a root ca, you might want to include a "Subject Key Identifier" AND an "Authority Key Identifier".
For self signed certificates they'll both include the same key ID but still it seems to be quite common for root CA's to do it.
XCA however fails to create a selfsigned certificate if "Subject Key Identifier" AND an "Authority Key Identifier" are requested:
> The following error occured:
> error:2207707B:X509 V3 routines:V2I_AUTHORITY_KEYID:unable to get issuer keyid