To be clear, the webapp does no calculations for extension boards. It simply requests the station names from the firmware. In the case of the interval program this information is stored in snames.txt and is retrieved by navigating to:
This should show you the snames object which is parsed for station names. This should be trimmed to the appropriate number of stations when sent (at least it is on the Arduino firmware).
Therefore, this might be a bug in the interval program returning everything in snames.txt without parsing out how many stations exist.
Try trimming your snames.txt manually and see if that fixes the issue.
Update: Looking a little closer, I suppose I could check the nboards variable on the /vs page and parse it myself. I haven’t in the past because I assume the firmware did this for me. Please let me know if this is something I should add/fix.