Redundant/broken shortcut (MS 4.5.2)
When you R-click, point to Select, and point to Similar, the Ctrl+Shift+A keyboard shortcut is still shown for the "More..." dialog box.
As far as I know, that shortcut is supposed to add an A to the selected note chord. What it actually does is insert an A of the same time value before the selected note or chord, adding a beat to the current bar without changing its time signature. (That can't be right, can it?)
Also, when you search for Ctrl+Shift+A in Preferences > Shortcuts, that shortcut is shown for both "More..." and "Insert A". Okay, over to you!
Comments
As far as I know, this shortcut "Select similar elements with more options" is not defined by default in Preferences/Shortcuts. At some point, you set it that way yourself :)
You can see this (and then change it to suit you) by selecting it and clicking on the tab at bottom right, Reset to default.
Add a note to a chord is done by using Shift+A (or other note).
Crtl+Shift+A is performing when the shortcut indicates, Insert note.