1 of 3

Why so many vocalized shvas in Siddur Tehillat Hashem?

I prayed using a Siddur Tehillat Hashem the other day, and I noticed that many more shvas were marked with an asterisk above, indicating that they should be vocalized, than I'd expect. In particular, there were many cases in which a shva following an unaccented "short vowel" and not under a letter with a dagesh got an asterisk.

One example, which I confirmed is not marked for vocalization in the Artscroll Siddur (for example), is the last word of the second verse of Psalms 104 (AKA "Barechi Nafshi"): כַּיְרִיעָֽה, which would be pronounced like "kayeriy'a" according to Tehillat Hashem and like "kayriy'a" according to Artscroll.

Where does this distinction come from?