After reading through some other threads, I believe this response on another thread may explain what is happening, but I’m not certain.
Forum thread about stimulus timing accuracy
The timing of visual stimuli must be a multiple of the refresh rate. Furthermore, simple keyboard responses are not measured on a millisecond basis. So my guess is that the ‘fixation to ROI’ and ‘keyboard response’ are being checked at different intervals, which results in slight differences in their end timing.
Since my data indicates that the onset of the response component and ROI components started at the same time, my hope is that I can simply adjust the ROI.timesOff to match the time of keyboard response if my ROI.timesOff records a time after the keyboard response.