Fig. 1 When a decimal tab behaves as decimal paragraph alignment, cell content aligns automatically to the decimal. Notice how the cell marker moves over automatically to the decimal tab position, just as it moves over to create center or right paragraph alignment.
Fig. 2 When you need to insert a tab character to push cell content to the decimal, the decimal tab is behaving as a tab and decimal alignment has not been correctly set.
People often mistake decimal paragraph alignment in Word tables for decimal tabs, and there is a good reason for that. After all, you need to place a decimal tab to create decimal alignment in a table.
However, decimal alignment and decimal tabs are not the same thing. When a decimal tab is behaving as a tab, it is necessary to place a tab character in front of the text to push it to the decimal tab stop. When the decimal tab is behaving as paragraph alignment, your text will automatically align to the decimal when that tab is placed.