The TL21 alligator clips aren't proper kelvin clips because both jaws are permanently electrically connected and both the pair of wires is just soldered to the same spot. The resistance between the jaws and DUT isn't compensated for. Thankfully the alligator clips are very stiffly sprung so any resistance is minimal.
I modified mine with proper kelvin clips and longer leads.
The TL22 tweezers have the same problem. That said, i haven't seen any proper kelvin tweezers.