Business Internet - 100mbps Down/ 20mbps Up but gets ~35-40mbps Down - Why?

In the initial ATT Internet installation, the tech (on his mobile device) ran speedtest and got 81.6 (down) / 22.3 (up) - 6-months ago.  I also ran speedtest and I got comparable results at that time.   I was good with the results, but more importantly, the initial 5-months of wifi / direct connect usage, we were happy... but now the speedtests are all over the place (37/29; 41/20; 32/15; 20/20; 15/9; etc), no apparent slowness or websites not being served.  After the 5-month period, we started getting slowness, can't get emails, or websites not being served - all periodic / sporadically occurring.  I called ATT and they sent a Tech (he couldn't resolve) and transferred the issue to another Tech (out-side line guys).  Out-side lines tech tested and claims all was good, but I was still getting poor to sometimes good service.  So at this point the issue still existed.  I called ATT and they had scheduled another Tech to return in 2-days to replace the ATT modem.  I waited 4-hours, then a message came to my mobile phone saying "Sorry we did missed you.  Still having trouble?  Text NO to close your ticket.".  Well, I texted "YES".  ATT text responded immediately with "Sorry we did not understand your response.  Text NO to close the ticket."  At this point, I continued to wait for an additional 5-hrs, hoping that the scheduled technician will either call me or come, but the Tech never showed or called me.  Very disappointed, since I waited a full day for the Tech to replace the ATT-modem, and hence on this Post.  My initial design/configuration on the ATT modem (Arris BGW210-700) was setup with IP Passthru Mode, since I had a Firewall between my users and the ATT modem.  I eliminated the Firewall and reconfigured the ATT modem back to not IP Passthru, and having the ATT modem perform DHCP Server function.  This new configuration was functional, so I ran the speedtests at various times again and the results (32.1/15.2; 34.7/20.2; 34.0/16.7 >> wifi with 3 different devices ~25-ft away; and ~10-ft away: 42.3/20.5; 95.1/19.5; 47.1/20.0; ) and ( 55.6/0.5;  69.6 / 20.1; 54.2/0.5 >> directly connected ran 3-tests with same laptop).  Since I have ordered and am paying for 110mbps-Down/22mbps-Up, I was expecting some degradation maybe 20-30%, but 50%+!!!!  Anybody have any similar experience?