The first time I noticed this was in spring of last year. I purchased the same shincha as a fellow forum member (Chip if memory serves). He noted the tea was too highly roasted to his taste, and I for myself found it reminded me of kamairi-cha. It is like something clicked, and I started paying more attention to this. At first I found his assessment of this particular tea a bit harsh, as this added taste was kind of "fun".
This firing will give the tea a certain sweetness, something along the line of faint baked pastry aromatics (Maillard reaction I guess). As long as it stays below the threshold where I consciously notice its presence, it probably gives the tea some dimension, some depth. But there can be too much of a good thing.
I recently went through a bag of sencha from Thés du Japon I had purchased before. My recollection and notes indicated this tea had great depth and clarity, aromas that were not very powerful, but were able to shine through because it was a tea that had no flaw. The first session with it this time I found it great, if roasted a bit much. Then the following sessions, once the excitement had died down, I found the roast was really bothering me. I checked Florent's assessment and, sure enough, it rated the tea 3 out of 4 on the "firing" scale. I cannot tell whether the tea was different last year, or whether I am different from last year, but I will surely pay more attention to this part of Florent's ratings from now on.