The difference, of course, is that one is a Latin word, and the other is an English phrase with the same meaning. But they're different words, so different tags. It's all completely rational and logical, and if you don't see the logic and/or reason, well: you're unworthy to write HTML. See Figure 1.

P.S. I love this ancient "see figure 1" meme. It's originated in the early 80s, and still as relevant as ever, and probably will be forever.

    Please stop submitting SPR's.  This is our system.  We designed
    it, we built it, and we use it more than you do.  If there are some
    features you think might be missing, if the system isn't as effective
    as you think it could be, TOUGH!  Give it back, we don't need you.
    See figure 1.

   ---------------------------
   !            -            !
   !           { }           !
   !           | |           !
   !           | |           !
   !        .-.! !.-.        !
   !      .-!  ! !  !.-.     !
   !      ! !       !  ;     !
   !      \           ;      !
   !       \         ;       !
   !        !       :        !
   !        !       |        !
   !        |       |        !
   !                         !
   ---------------------------
      Figure 1.