הַסַּבְתּוֹת -- Hasavtot?

So walking Meir to my parents yesterday, I noticed a new storefront on Main Street. A big sign advertised that soon a store called "The Grandmothers" would be opening up. In Hebrew, "Hasavtot" --הַסַּבְתּוֹת (though the sign did not possess the nikkud).

I was a bit surprised , but apparently this is the modern Hebrew term. What was surprising to me was this:
סַבְתָּא is Aramaic, as we can tell from the tav kametz aleph ending. It is the feminine singular for grandmother, both in the definite (=the) and absolute form. (It is really the definite form, but Babylonian Aramaic makes no distinction between the two.)

As such, we should expect that to pluralize it, we would use the definite plural of Aramaic, and we do this by changing the sheva to a kametz:
Thus, סַבָּתָא would be plural with either absolute or definite article, and would be what I would expect for "The Grandmothers."

What appears to have happened is that the kametz aleph (without the tav included) ending was interpreted as feminine singular of Hebrew, as if it were kametz heh. The feminine plural was then be produced by eliminating the kametz aleph (=kametz heh) ending and replacing it with cholam melei thav. Since they do not recognize this word as already in possession of the definite article (since this is an Aramaic feature), they add a heh patach gemination to get "the."