One way to hold this word: you worked something out in your head, and the answer came out wrong. Picture someone counting cups for a party and forgetting half the guests. The counting was real, the effort was real, and the number was still short. The word names that wrong answer, not the person who reached it. That is why it moves so easily from numbers to judgment, because both are answers you worked out and both can be short.
From the word’s history
The packet's source gives the make-up plainly: mis- plus calculation. Mis- is the prefix English uses for doing something wrongly, and calculation is the working-out itself, so the word is simply a working-out that went wrong.
來源把組成寫得很清楚：mis- 加上 calculation。mis- 是英文用來表示「做錯了」的字首，calculation 就是那個「算」的動作。所以整個字就是「算錯了的那一次計算」。
Wiktionary entry supplied in this packet (kaikki:3f6f69a190036ae4:1449203728:9394): 'From mis- + calculation.'
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