Measuring RMS Voltages: How true is your True RMS multimeter really?

I noticed some strange RMS values with my "True RMS" multimeters when measuring a rectangular wave form at different duty cycles which wasn't what the wave form equations predicted. I always thought that a "True RMS" meter could handle pretty much any wave form without me having to worry about its shape, etc. Well it turns out this is not the case. ... https://www.youtube.com/watch?v=jhy_kfhwwbo