What makes sencha karigane sweeter than sencha?

Why does the addition of stems and veins to the tea increase the sweetness of the brew? Is there a specific chemical in the stems and veins of tea leaves that's responsible for the change in flavor palette? If not, what's the mechanism by which adding stems and veins seems to change the flavor?

I've tried many sencha and sencha karigane over the last few years, and it seems that the karigane teas consistently have a lighter and sweeter flavor than sencha without stems. This is in fact what karigane sencha is supposed to be by comparison - nearly every tea site that sells karigane tea makes this claim of "sweetness," and it's quite accurate.

Several sites seem to say that karigane stems add theanine to the tea, which contributes sweetness. As one example:

Karigane stems are rich in Theanine, which is found in Gyokuro and Matcha grown under shades twenty to thirty days before the harvest. This higher concentration of Theanine gives the tea a sweeter and less bitter flavour than standard Sencha.

https://cominstea.com/products/sencha-karigane

However, there doesn't seem to be much direct information on the flavor of theanine to corroborate this directly. In fact it seems like, if anything, the addition of theanine should add savory flavors to the mix, not sweet ones. However, karigane sencha is most distinctly not umami per se - where tea like gyokuro or a premium sencha can have a clear and distinct umami palette. Karigane teas almost always taste markedly sweeter (at least to my taste!).

This may be due to my own misunderstanding of the literature, and how flavors emerge. But it also may be due to a marketing miscommunication, or something of that ilk. So, I'm curious what the actual facts of the matter are.