Hmm, yeah, without further information, it's hard to say for sure. I'd speculate that those serious adverse events deemed vaccine-unrelated probably occurred in patients with advanced cancer. Such patients are expected to experience adverse health consequences anytime.

Whereas for the serious adverse events deemed vaccine-related, they probably occurred in patients with early cancer who shouldn't be experiencing serious health consequences anytime soon.

But I see your point, such decisions are often subjective. Physicians are more cautious in assigning an adverse effect to vaccines due to the ambiguity.

For instance, if people or lab rats get covid or cancer, you see the negative effects easily. But if they get vaccinated, we almost never see them.