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The label of the one-pound box of Cohen Halperin Shmura Matza reads:
משקל כל מצה כשיעור כזית
In English (with some Hebrew thrown in, in transliteration, of necessity), that's:
The weight of each matza is the measure of a k'zayis.
Isn't k'zayis a volume measure?
There is a famous machlokes whether kzayis is a volume or weight measure. Many sefardim follow the Kaf Ha'Chaim that it is a measure of weight.
http://www.vbm-torah.org/pesach/pes62-yr.htm
...As the Maggid Mishneh explains, the Rambam here establishes that these measurements are determined by volume, not weight, a principle that seems to emerge as well from the mishnayot (Keilim 17:6 and elsewhere) and many other sources. This is also the view of the Mishna Berura (486:3). However, the Kaf Ha-chayim (168:46) observes that common practice was to calculate according to weight, seemingly against the straightforward reading of all the poskim. We may perhaps attribute this practice to the difficulty involved in constantly measuring volume.
Today, Ashkenazim calculate volume, in accordance with the aforementioned ruling of the Mishna Berura, and this is also the oft-cited view of Rav Yosef Shalom Elyashiv. The Sefardic authorities dispute the issue: Rav Ovadya Yosef (Yechaveh Da'at 1:16) writes that the weight measurement should be used, while Rav Shalom Messas (Tevu'ot Shemesh) maintains that we determine measurements based on volume.
You are correct. The person in charge of the text of their labels is either ignorant in areas of halachah lemaaseh or pirush hamilim, or is a Sefaradi (who does not hold like Rav Ovadiah Yosef nor like Rav BenTzion Abba Shaul and) who is unaware of the Ashkenazic practice.
While the explanation of the label is probably just as Yahu said, there is something more to the general question, "Isn't k'zayis a volume measure?". In Rav Slifkin's now-famous Evolution of the Olive article, he spends a while toward the end describing a historical phenomenon in which, for the sake of simplicity and standardization, people began measuring matza by weight in stead of volume - a practice, which he too believes to be a mistake.
In this Halacha (Orach Chaim 486:2), the Shulchan Aruch HaRav clearly indicates that he considers a K'zayit a measurement of volume, not weight.
There, he says that the empty space between the Maror leaves cannot be counted as part of the K'zayit. On the other hand, if the item has no empty spaces, it doesn't matter how airy (and therefore lighter in weight) it is.
I was recently told in the name of R' Shusterman (Chabad Posek/Rav and Shliach to Beverly Hills) the following:
In the Luach Colel Chabad, R' Avraham Chaim Na'eh writes that a K'zayit Matzah is the weight of one ounce. The Lubavitcher Rebbe told his Chassidim many times to follow what it says in the Luach Colel Chabad, and proofread it. Since the Rebbe was aware that the Luach said we consider the weight of the Matzah, that is the way Chabad Chassidim should measure a K'Zayit of Matzah.
(I did not hear if this was to apply to a K'Zayit year-round as well, although it would stand to reason that it would).
According to many poskim both Ashkenazi(e.g. Rav Eliashiv) and Sephardi (e.g. Kaf HaHayim) it is both. A Kazayit is often measured in x-number of cubic centimeters, which correspond directly to x-number of grams, as 1cc=1gm. Rabbi Slifkin did a wonderful article on it that explores the halakha of both Sephardim and Ashkenazim (of which he is one).
