It could simply be a heavy chassis, more heat sinks etc., or their specs are not accurate. We already know reportedly Monolith might have misstated the transformers VA rating for the 5X200 W model so what else they might have missed? If they did, they have not corrected it and still say:
Toroidal Transformers2 (1,230VA and 1,025VA)
Weight is an indicator of the robustness of the power supply but not with good accuracy for a variable of reasons. By the way, the monolith 7X200 specs say the weight is 93.2 lbs, the ATI1800, also specified 7X200 FTC FBW at 8 ohm, weighing 93 lbs according to the manual and the spec sheet, and 95 lb according to web page.
https://www.monoprice.com/product?p_id=14566 (here it says 93.2 lbs)
https://www.ati-amp.com/AT1800.php (says 95 lbs)
https://www.ati-amp.com/manuals/ATIpowerampspecs.pdf (says 93 lbs)
So the Monolith may actually weigh 0.2 lbs heavier, but again, I wouldn't take those numbers to the bank, and 0.2 lb difference is nothing conclusive anyway obviously. Your 10 lbs difference may be from comparing the net weight of the Monolith to the shipping weight of the ATI.
Maybe you can also find out the transformers total VA rating for the 7 channel version of the AT1800. That would be more meaningful and useful than the weight imo..