I suspect it’s by construction, looks like they’re offering every permutation of treat choice twice.

Below, I filtered for A vs E, the top two choices. Notice how they switch left and right hand each time:

A/E :: A

E/A :: A

A/E :: E

E/A :: E

A/E :: E

Author here, yes, that's correct.

The initial set-up was to do 3 comparisons of the 5 treats (30 trials), alternate between right and left hand, then write a quick python script to randomize the order.

A bit more than halfway through the experiment, I ran the model and realized that A/D/E were the only contenders left, so I removed the B/C trials and added more A/D/E trials.