It explains at some point that Spike refers to Angel as his sire because he sired Drusilla and Drusilla sired him, so he’s like the main sire person, there is no lack in continuity there. 

It explains at some point that Spike refers to Angel as his sire because he sired Drusilla and Drusilla sired him, so he’s like the main sire person, there is no lack in continuity there.