[請益]there must have been VS there had to be

[tucson (tucson)]

A在B對訴說昨晚的事情:

A: I could hear music coming from the house. There must have been somebody at
home.

請問為何用 There must have been somebody at home.

而不用 There had to be someone at home.
???

這句 There must have been somebody at home.

若改成 There had to be someone at home.

在意義上或使用時機上有無不同?

Thank you

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→ allenh0728: must是強調說一定有
→ allenh0728: had的語氣比較沒有像那樣