It’s not clear how you’re reading your result. But characters 7 thru -1 of myText returns a list of the characters. If you’re coercing this to text and you happen to have left AppleScript’s text item delimiters set to a space, the space will be inserted between each list item in the result.
The safest and most efficient way to extract the substring is set myText to text 7 thru -1 of myText