PennController uses an internal counter to counterbalance group assignment. By default, the counter increases by one at the end of an experiment, which rotates which experimental item group is run.
11 and 13 of your participants opened the link to your experiment when the counter had a value whose modulo 7 was 1 or 4, respectively
You could use
SetCounter to insert a trial at the very beginning of your experiment to increase the counter at the very beginning instead, but that won’t ensure that your participants start with a different counter value (eg. if multiple people click the link at the same time) or that participants with a different counter value will complete your experiment (anyone can decide to leave before finishing)
The most certain way to collect an even number of participants is to collect your data in batches, circulating a link with
?withsquare=0, one with
?withsquare=1, one with
?withsquare=2, etc. and run more participants for each link as needed after you check your results file