AppleScript: “Repeat” Loop Only Works With Variable

Very curious as to why this works (the context is an open and activated/frontmost Safari window displaying the "Customize Toolbar" sheet):

tell application "System Events"
    tell process "Safari"
        set entireContents to entire contents of sheet 1 of front window
        repeat with x in entireContents
            if class of x is image and description of x contains "My Extension" then
                set targetExtIconPos to position of image 1 of x
            else if class of x is image and description of x is "default set of toolbar items" then
                set defaultToolbarPos to position of x
            end if
        end repeat
    end tell
end tell

but changing repeat with x in entireContents to repeat with x in entire contents of sheet 1 of front window throws an error (and does so even if parenthesis are utilized as in repeat with x in (entire contents of sheet 1 of front window))?

The error I get is: execution error: System Events got an error: Can’t make item 1 of entire contents of sheet 1 of window 1 of process "Safari" into type specifier. (-1700)

This caused alot of headache before I randomly found it would work if I just assigned it to a var first…

Also is there a way to achieve what I am attempting here without using a loop at all? I feel like there is a way, but the syntax is just awfully hard and complex to figure out and nail down.