Krupski:
Why repeat the experiments? I think they showed reasonable data. In fact, they proved "the point"... yes there is an
Us mortals do not get to play with this sort of equipment normally. 
Krupski:
Also, what nobody seemed to mention (although I may have just missed it) is that the output drivers for the AVR pins, every time they switch, go through a very short period where they are BOTH conducting and placing virtually a short circuit across the Vdd supply (hence the need for bypass capacitors).
Shoot through ?
Also spice models would suggest a short high current pulse on the transition.
I suspect breadboard layout problems will make a nonsense of the measurement results anyway.
Resistor may slow down switching but around here i think it best to suggest it as it gives some protection to the outputs from the inevitable silly mistake that the newbies (and others) make.