I had the same problems with my signatures. Parts of them kept showing up in Times when I had them formatted in Arial. This was driving me crazy but I think the problem is Word is the email editor but when it comes to email signatures Word and Outlook don't quite sync and, maybe, revert to the typeface which is the default font in Word if there's any confusion. Anyway...
After 2 hours of being driven absolutely mad by this, here is what I did and it worked.
I closed outlook (File, Exit) and all of the addins to it. (Wireless sync and a phone add in).
I opened Word, Tools, Options, General Tab and then Email Options (at the bottom right) then Email Signatures.
I deleted the offending signature and closed the menu.
Then in word I changed the default type to Arial. Word, Format, Fonts, selected Arial, then Default at the bottom.
Then I created the signature in Word just as I wanted it, selected it, and cut.
Then back to Tools, Options, General, Email Options, Signature and select New Signature and pasted what I had just created in Word.
Then scroll your cursor over "Select the signatures associated with." which is below and greyed out then click and you will get a message that says something like: this can't be done without opening outlook; do you want to open outlook? Click yes and then you can select when you want to use this signature.
Hope this helps.