Why are my tab spaces not uniform?

Hi, I have a question about the tab spaces at the beginning of paragraphs.

In the Word doc, I set the paragraph tabs to 0.50 cm and in Sigil I'm using three spaces.

However, the spaces are not uniform throughout the whole document. Sometimes the space before the first word of the paragraph is larger, although the code is exactly the same. See screenshot for what I mean.

Can anyone help?

Thanks,
Helen

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