In MS Word, decimal tab stops affect paragraph layout in table cells even if tab character is not present. The reasoning is that it's hard to insert tab symbol in table cell, as pressing Tab navigates to the next cell (Ctrl+Tab should be used). 

When decimal tab stop is present for a Paragraph, and the paragraph is in a TableCell, the content of the paragraph should be aligned to the decimal tab stop (analogically to the case when the Paragraph is not in a table cell and it contains tab character.