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Originally Posted by xander838  when spike refers to Angel as his sire, it's in the cintext of he taught him everything he knows about life as a vampire, it was made clear long before season 7 that it was dru that sired Spike |
The simple explanation is that Spike was originally intended to be there just for season 2. His backstory - that he was sired by Angel - wasn't that important but fitted in with the season quite well.
But when the character became so popular that they wanted to bring him back, they reworked his backstory with a lot more attention to detail.
There's no way to resolve the difference in terms of plot - there's no doubt that when it was first written they meant Angel to be Spike's sire - they (the writers) just changed their minds. But it wasn't a mistake per se, it changed because of the new demands of the character.
The Buffy universe developed over time, it didn't spring full-grown from Wheedon's mind. There was bound to be the odd discrepancy here and there.