The four parshas of tefillin appear slightly differently each time when the Torah describes the mitzvah.
וְהָיָה֩ לְךָ֨ לְא֜וֹת עַל־יָדְךָ֗ וּלְזִכָּרוֹן֙ בֵּ֣ין עֵינֶ֔יךָ לְמַ֗עַן תִּהְיֶ֛ה תּוֹרַ֥ת יְהוָ֖ה בְּפִ֑יךָ כִּ֚י בְּיָ֣ד חֲזָקָ֔ה הוֹצִֽאֲךָ֥ יְהֹוָ֖ה מִמִּצְרָֽיִם׃
“And this shall serve you as a sign on your hand and as a reminder on your forehead—in order that the Teaching of the LORD may be in your mouth—that with a mighty hand the LORD freed you from Egypt.
וְהָיָ֤ה לְאוֹת֙ עַל־יָ֣דְכָ֔ה וּלְטוֹטָפֹ֖ת בֵּ֣ין עֵינֶ֑יךָ כִּ֚י בְּחֹ֣זֶק יָ֔ד הוֹצִיאָ֥נוּ יְהוָ֖ה מִמִּצְרָֽיִם׃ (ס)
“And so it shall be as a sign upon your hand and as a symbol on your forehead that with a mighty hand the LORD freed us from Egypt.”
וּקְשַׁרְתָּ֥ם לְא֖וֹת עַל־יָדֶ֑ךָ וְהָי֥וּ לְטֹטָפֹ֖ת בֵּ֥ין עֵינֶֽיךָ׃
Bind them as a sign on your hand and let them serve as a symbol on your forehead;
וְשַׂמְתֶּם֙ אֶת־דְּבָרַ֣י אֵ֔לֶּה עַל־לְבַבְכֶ֖ם וְעַֽל־נַפְשְׁכֶ֑ם וּקְשַׁרְתֶּ֨ם אֹתָ֤ם לְאוֹת֙ עַל־יֶדְכֶ֔ם וְהָי֥וּ לְטוֹטָפֹ֖ת בֵּ֥ין עֵינֵיכֶֽם׃
Therefore impress these My words upon your very heart: bind them as a sign on your hand and let them serve as a symbol on your forehead,
All four cases the Torah refers to the arm tefillin as being a sign.
Why is only the arm tefillin referred to this way and NOT the head tefillin- which instead is referred to as totafos or a remembrance?
Edit: I guess you could conversely ask why only the head tefillin are called a remembrance or totafos, but I don't see this as as big a question. According to Menachem ibn Seruk (cited by Rashi) and the Ibn Ezra on Shemos 13:16, totafos means remembrance. Rashi explains that you'll see the head tefillin, talk about it, and remember the miracles of Egypt. This wouldn't apply to the arm tefillin, which are supposed to be covered. As well, according to the other explanation that Rashi brings, that totafos refers to the four parshiyos, this could apply more to the head tefillin which have four compartments. (Although you could then ask why only the head tefillin have four compartments, but that's probably too tangential to be a relevant question)