Simple. The strain from the loop wanting to straighten out tends to resist force that wants the loop to close. Tighten the loop and the strain increases.
Besides, I make two marks (after the photo was taken) so I could tell easily of it did. Because it's a zip tie, the loop can only close and not open.
For the record, the dipstick (seen in the photo with the yellow plastic end) is the same color as the oil (I believe it's a chromate coating), its surface does not cause the oil to form a stable meniscus, and it has a shiny surface, all combining to make it next to impossible to read with clean oil. The nylon has none of these negative characteristics.