The person fellating receives the ejaculate and uncertain and in a panicked state of mind is unsure as to weather spit or swallow. This could be the result of conflict between liking and loathing the tasting of sperm, the pressure to please the felatee, or, the receiver of sperm is unsure as to weather the sperm is in fact swallow-worthy. In any case, some unintentionally gets ingested either way. Usually with a mass of spit.
"I hear you've got a new bird. Does she swallow?" "Yeah, usually, but last night I ate a curry and she hates the taste of curry in my splooge. So, she spallowed instead."