Well as the official ISP Whipping boy on the forums let me explain this in simple terms to everyone experiencing this issue.
It does not necessrily have anything to do with you network, nor does it entirely depend on what you are doing on your PC
There are various factors that can influence gaming latency negatively:
Client Side Activity
This is the download or update issue some people were referring to
Throttling
This includes line speeds being throttled under uncapped terms and conditions of usage, check your bandwidth limit and confirm with your ISP if you think this may be the cause of increased latency
Exchange Activity
This is the physical exchange your line conencts to, it may be under strain due to larger amounts of lines being connected to it, or more traffic than usual being routed through it, if you think this might be the issue phone up your line provider and ask them to do a throughput test on your exchange.
Network Issues
This includes but is not limited to , large scale WAN issues, lines going down across the country over the past few weeks, Seacom's break, Fiber lines getting severed in random blasting incidents surrounding the Gautrain etc.
Network Activity
This includes issues that your ISP may or may not have with routing, seeing as the Webafrica boys decided to chime in here let me add this to the list, Webafrica specifically have issues at certain times , these have been documented in the past with connections to local gaming servers not ON their network, routing is a major contributing factor here, and is your ISP does not route correctly this may severely impact connectivity and overall latency.
So to conclude, no your router in fact does not contain the entire Internet, you are part of a dynamic, fluctuating system that interconnects on numerous levels with other dynamic systems.
And btw, Necuno, you keep on focusing on one point, let me clarify it for you though
90% of users issues regarding latency during gaming can be attributed to the client machine downloading something in the background, this includes Windows updates, Steam Updates, Xfire updates etc.
Next time you feel like posting in a help topic, why not actually try and help ?