Why mention that one sign is insufficient?

Summary: Bechor Shor explains that there is indeed a madreiga in even one sign.

Post: From parashat Shemini:

4. But these you shall not eat among those that bring up the cud and those that have a cloven hoof: the camel, because it brings up its cud, but does not have a [completely] cloven hoof; it is unclean for you.		ד. אַךְ אֶת זֶה לֹא תֹאכְלוּ מִמַּעֲלֵי הַגֵּרָה וּמִמַּפְרִיסֵי הַפַּרְסָה אֶת הַגָּמָל כִּי מַעֲלֵה גֵרָה הוּא וּפַרְסָה אֵינֶנּוּ מַפְרִיס טָמֵא הוּא לָכֶם:
5. And the hyrax, because it brings up its cud, but will not have a [completely] cloven hoof; it is unclean for you;		ה. וְאֶת הַשָּׁפָן כִּי מַעֲלֵה גֵרָה הוּא וּפַרְסָה לֹא יַפְרִיס טָמֵא הוּא לָכֶם:
6. And the hare, because it brings up its cud, but does not have a [completely] cloven hoof; it is unclean for you;		ו. וְאֶת הָאַרְנֶבֶת כִּי מַעֲלַת גֵּרָה הִוא וּפַרְסָה לֹא הִפְרִיסָה טְמֵאָה הִוא לָכֶם:
7. And the pig, because it has a cloven hoof that is completely split, but will not regurgitate its cud; it is unclean for you.		ז. וְאֶת הַחֲזִיר כִּי מַפְרִיס פַּרְסָה הוּא וְשֹׁסַע שֶׁסַע פַּרְסָה וְהוּא גֵּרָה לֹא יִגָּר טָמֵא הוּא לָכֶם:
8. You shall not eat of their flesh, and you shall not touch their carcasses; they are unclean for you.		ח. מִבְּשָׂרָם לֹא תֹאכֵלוּ וּבְנִבְלָתָם לֹא תִגָּעוּ טְמֵאִים הֵם לָכֶם:

R' Yosef Bechor Shor writes:

כל אלו כי משמשין בלשון אם, כלומר, אם יש להם סימן אחד, כיון שאין להם האחר אסורים, והוא כגון אע״פ, ואפילו הבא בסימן אחד טמא, כגון חזיר וגמל ושפן וארנבת טמא[ים] ואפילו הגוים אין רגילין לאכל אלא אותם הבאים בסימן אחד,
 שהאחרים מזוהמים יותר מדאי, ובזה יש להוכיח לגוים מנהגם, שמי שאין לו סימן מזוהם.

"All of these instances of כִּי function in the sense of אִם, that is to say, 'if they have a single sign, since they do not have the other sign they are forbidden.' And even that which only has a single sign is impure, such as the pig, camel, hyrax and hare are impure. And even the gentiles are only accustomed to comsume those which have {at least} one sign, for the others are exceedingly disgusting. And in this {last point} there is to prove to the gentiles {via} their custom, that one who does not have a sign is disgusting."

In terms of the last statement, I suspect that the reference is to Christian polemics regarding circumcision.

If not for this explanation, some might have thought that the reason for this stress is similar to that of the Holy Hand Grenade:


Alternatively, ambiguities in Biblical Hebrew might have otherwise misled people to think that either sign was acceptable, or else they might have lacked scientific knowledge and believed, based on many many instances, that these two signs always co-occur.