Download speed going crazy

my download speed is about 25 kB/second...
But right now I pressed Software Update and I did a download of the Security Update 2009-004 and Safari 4.0.3. And everything ended in less than 5 seconds!!! Then I restarted and I checked again for updates and it said "You are currently running on the latest software." I dont know if I should be freaking out or if I should be calm...
Please...advice

It's a question of what you are downloading and from where.
Apple's updates always come down at full speed because they are hosted on a massive server with huge upload speeds, so in theory if you have 10 meg broadband you will download at 10 meg (give or take~).
If you are downloading form sources with less bandwidth, you get slower speeds.