Check your MS Word Styles. Make sure no direct formatting.
Also no headers, footers, page numbers or automatic hyphenation (all those are only for paper / PDF export)
No inches or cm. Only pt (or em if your version of Word supports it. 1em = 12 pt for ebooks, but not for paper!).
Make sure on your dialogue that each part equals one paragraph. Do not use Shift Enter or similar, but just enter. A "shift enter" may translate to a <br />
Should be
<p class="some-body">Arriving in San Francisco we were transported to the Victorian home we had arranged to rent in Pacific Heights located high on a hill overlooking the city. We were entranced by the sparkling waters of the bay and I remarked it wouldn’t be surprising if someday they figured out how to build a bridge across there.</p>
<p class="some-body">“Something’s shaking...”</p>
<p class="some-body">“What’s that rumbling noise? Sounds like...”</p>
Don't enter blank lines to space, instead have a paragraph style that adds non-zero top (& optionally bottom) margins.
Have the regular body text style for narration and dialogue have a first-line-indent value. A paragraph style for after a heading or scene break can have a zero first line indent margin.
Don't set any line-spacing