Because it use less instructions.

code/controllers/master_controller.dm

@@ -252,7 +252,7 @@ datum/controller/game_controller/proc/process()
 			Disease.process()
 			continue
 
-		active_diseases = active_diseases - Disease
+		active_diseases -= Disease
 
 
 /datum/controller/game_controller/proc/processMachines()
@@ -276,7 +276,7 @@ datum/controller/game_controller/proc/process()
 			Object.process()
 			continue
 
-		processing_objects = processing_objects - Object
+		processing_objects -= Object
 
 /datum/controller/game_controller/proc/processPipenets()
 	last_thing_processed = /datum/pipe_network
@@ -286,7 +286,7 @@ datum/controller/game_controller/proc/process()
 			Pipe_Network.process()
 			continue
 
-		pipe_networks = pipe_networks - Pipe_Network
+		pipe_networks -= Pipe_Network
 
 /datum/controller/game_controller/proc/processPowernets()
 	last_thing_processed = /datum/powernet
@@ -296,7 +296,7 @@ datum/controller/game_controller/proc/process()
 			Powernet.reset()
 			continue
 
-		powernets = powernets - Powernet
+		powernets -= Powernet
 
 /datum/controller/game_controller/proc/processNano()
 	for (var/datum/nanoui/Nanoui in nanomanager.processing_uis)
@@ -304,7 +304,7 @@ datum/controller/game_controller/proc/process()
 			Nanoui.process()
 			continue
 
-		nanomanager.processing_uis = nanomanager.processing_uis - Nanoui
+		nanomanager.processing_uis -= Nanoui
 
 /datum/controller/game_controller/proc/processEvents()
 	last_thing_processed = /datum/event
@@ -314,7 +314,7 @@ datum/controller/game_controller/proc/process()
 			Event.process()
 			continue
 
-		events = events - Event
+		events -= Event
 
 	checkEvent()