It helps if you can divorce score from execution when evaluating your performance. Focusing heavily on score takes focus away from where it needs to be: proper execution.
Even so, there is always the confidence/expectation factor. It is natural to be disappointed when performance doesn't meet expectation. If the disappointment is allowed to undermine confidence, the effects can be even more devastating.
As some of you have pointed out, being excited about bowling can be counter-productive. Using a little meta-cognition (thinking about our thinking), we'd probably realize that on those nights where we were excited, we were probably focused on enjoying the moment, the lanes, the physical act of bowling, and so on. Maybe we "gloss over" the concentration needed on a shot-by-shot basis in those situations.
Like you, ~9, some of my very best outings have been while physically sick. I believe that when our body wants to shut down, our conscious thought is more inclined to settle on a focus on a single thing to the exclusion of others. (sort of a quiet, self-preservation mode) When that single thing is the bowling, look out.
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J.J. "Waterola Kid" Anderson, the bLowling King : Kill the back row