It's because setting an in-point is inclusive (the frame selected will be in the trimmed clip) but the out-point is exclusive (the frame selected will NOT be in the trimmed clip). This makes a certain amount of sense from a mathematical point of view - if you want a 50-frame clip, you select your in-point, then jump ahead 50 frames and selected your out-point.
Avisynth's in- and out-points are both inclusive, which is why trim(0,100) produces a 101-frame clip, not a 100-frame clip. That feels less sensible to me.
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