You have probably copy/pasted the text contents from another formatted source (such as another website or a word document) which leads to the original formatting being preserved and transferred. Rich text editors in general try to preserve the original formatting when a text is pasted, and this will of course override the formatting you have chosen for the rich text fields in Muse, since technically the embedded formatting is applied on the top of the styling in Muse.
The way to prevent this is to first get rid of the formatting of a text before pasting it into the blog. On windows you can first paste the text into the NotePad (the plain text editor) as this will neutralize the formatting. Then you can re-copy the text from there and paste it into the blog. On a mac you can use the shift+alt+cmd+V shortcut (instead of cmd+v) to paste the text as a plain text, without transferring the formatting.
For the pre-formatted texts which were already pasted into the blog please select them all in the editor, then click the "Tx" icon (on the editor). This will try to neutralize the formatting and will work in the most cases.