In Sefer Shemos perek 13:9 Rashi writes:
על ידך ולזכרון בית עיניך. שֶׁתִּכְתּוֹב הַפַּרְשִׁיּוֹת הֲלָלוּ וְתִקְשְׁרֵם בְּרֹאשׁ וּבִזְּרוֹעַ:
That you should write these parshiyos and tie them on the head and the arm. Shouldn't Rashi have written that one should tie the arm tefilin and then the rosh like the halacha is stated and how they Torah mentions it? Why did Rashi write it like this?
Gam Ani Odcha - Rabbi Chaim Kanievsky answers based on Menachos 34b that the Shel Rosh is more Chashuv than the Shel Yad therefore Rashi mentions the Shel Rosh first.
תשובה: משום חשיבותה, עיין מנחות ל״ד ב׳. [בגמרא שם איתא, תפילה של יד עושין אותה של ראש, ושל ראש אין עושין אותה של יד, לפי שאין מורידין מקדושה חמורה לקדושה קלה. ע״כ].
Shouldn't Rashi have written that one should tie the arm tefilin and then the rosh like the halacha is stated and how they Torah mentions it?
Why did Rashi write it like this?
Answer: Rashi did write this to outline that the verse does not provide a source for the order of tying tefilin, that the source of this is not in the verse but a principle learnt out of the verse. This is the opinion of Rashi but Rabenu Tam indeed thinks that the verse teaches us the order of tying.
Explanations:
This Rashi is very interesting, and even more than interesting after the learning of the Gemara in Yoma 33b, this resolve partially the kushia of Tosfot on Rashi there.
See Gemara Yoma daf 33ab (and then Rashi, Tosfot, Ritba).
(33a) for Resh Lakish said: 'One must not forego the occasion of performing a religious command.
(33b) Said Raba: From what Resh Lakish said we infer that it is forbidden to forego the arm in favour of the forehead. How shall he do it? From the arm [he shall] proceed to the forehead.
Rashi explains that the Gemara said the importance to tie first the hand-tefila, because you meet firstly the arm, and secondly the head with your working hand.
Tosfot (the explanation is more comprehensive in Ritba) shows from the Gemara Menachot (36a) that from one verse we learn to ty first tefila of head and from another verse, to leave first tefila of head (totafot ben enecha when they are plural, that is head + arm), the tefila of head is present, i.e. there is no situation in which you have tefila of head only.
Here is a quote of Gemara Menachot 36a:
One taught: When a man puts on the tefillin, he should put on first the hand-tefillah and then the head-tefillah, and when he takes them off, he should take off first the head-tefillah and then the hand-tefillah Now it is right that when he puts them on he should put on first the one on the hand and then the one on the head, since it is written, And thou shalt bind them for a sign upon thy hand, and then it says, And they shall be for frontlets between thine eyes; but whence do we know that on taking them off he should first take off the one from the head and then the one from the hand? - Rabbah said, Rav Huna explained it to me.
Yet we can discuss the opinion of Rashi. Without deep explanation, we see that without the teaching of Resh lakish that Ein Maavirim al hamitsvot, despite the verse, we can believe to tie firstly the head-tefilin (because it is more Kadosh as soon in the answer of @Gershon Gold, or because you find it first in the bag). As you assume, the source of precession of the hand-tefilin according to Rashi is not in this verse. We can see in the Rashi on the verse an answer to the adtonishment of Tosfot. I wrote here a minimum of explanation, assuming that a comprehensive explanation of the Gemara and Tosfot Yome can be lenght.
Just an additional statement.Gemara (RL in Yoma) makes a comparaison between 2 situations:
Rashi feels that this Gemara is a proof that the verse quoted in the OP is not an intrinsec proof that we have to wear hand-tefila before head-tefila.
Note. All this reasoning can be rejected. We can say that there is a positive point of view explained in Masechet Menachot, to follow a specific order {1: hand, 2: head}.And there is a negative side, a prohibition to forego the hand in favour of the head.
