some people cancel last minute, so if you keep on checking or get lucky you're able to snag those apointments.
Yes, that is exactly what happened. I checked Tuesday evening and it said no appointments for next 12 months, refreshed a few times periodically but nothing useful came up (only random appointments a few months away and I'm traveling in August). I checked Wednesday morning and same no appointments available but refreshed a few times over the next hour and 2 appointments opened up, 1 same day for 11:45 and the other for that Friday. This was about 10am so I only had < 2 hours until the appointment but I WFH, didn't have any meetings scheduled for that time, and live within 15 min of EWR so was able to take the appointment on such short notice. They didn't even have me on the "list" when I went in but I just told them I scheduled it this morning and they were fine with that.
Overall if you refresh periodically appointments do open up but they go fast so you need to be quick to book of the time works for you. You can always schedule it and then keep on looking for a better time and reschedule if it comes up.
For my wife's appointment it was a little different, I was refreshing as I did for mine and after a while a whole block of appointments opened up for evening appointments, from 4pm - 10pm on Wednesday and Thursday for a few weeks in July. It shows at first 40+ available for each of those days and every refresh the number kept dropping. EWR on their website shows they are not opened in the evening so hopefully it's a legit appointment (didn't happen yet), I guess I'll find out soon.