text items returns a list, in this case three items: {“”, “”, “bbccbb”} where the first empty quote is the first “a”, the second empty quote is the second “a”, and the third part is the rest. By converting text items to text you get the whole list.
set AppleScript's text item delimiters to item 1 of "aabbccbb"
set td to AppleScript's text item delimiters --> "a"
set y to text items of "aabbccbb" --> {"", "", "bbccbb"}
I don’t know why the conversion reveals the “a"s. Seems to me the answer should be " bbccbb”