On the emails you only have seven available point sizes - usually 8,10,12,14,18,24 and 36. This was the same in Outlook Express. When you select a font size from the Tools/Options menu it will default to the nearest possible lower option from the available seven. So if you set it to 20 it will use 18 etc.
In answer to you final question it will not make any difference. Microsoft only considered it necessary to have seven font sizes in an email, whereas in Word you can select any size you want - even half sizes such as 12.5