I checked the javascript in index.htm. it makes a json call to get the data, which comes in an array. And the service handler in arduino gets the data for the requested channel from the correct array index.
if you got time now, you can test this by configuring 4 pumps (enable pwmb). configure master, antisync, sync, antisync, then let it run for a minute, then from a browser, enter
http://<yourcontrollerip>:8000/pwmpumpdata.json
and see what data it returns.
first set is master, second set should be complement of first set (255-first value, actual calculation is a bit more complex, as it takes into account the interpolated max, so it does not always use 255), third should be same as first, fourth should be the same as second.</yourcontrollerip>