For "for" statement with let V8 generates two scopes with i (inner for actual "let", outer to carry over value between iteration) and this looks strange from user perspective, and DevTools should hide outer scope.
More discussion on this could be found here: https://codereview.chromium.org/1653083002/

Example of this behaviour could be found in  v8/test/mjsunit/debug-scopes.js  at the test "Scope positions in for statement".