There are many different reasons:
1. Hardware components (installed RAM, type and size of your hard drive, type of processor, motherboard and graphic video card).
2. Windows OS components (they are many different versions of drivers included with OEM versions compared to just using a custom built system with default Microsoft drivers or installing drivers from the CD that came with your motherboard.)
3. Resources used by the software (number of processes, services and drivers) how much they need to consume to function.
In the past amount of RAM and CPU consumed by the software was the way you could tell if a product was going to run light or heavy. But that no longer applies to many software like security products.
Some products many consume a lot of RAM and CPU and still not effect the system response time while others that used the same amount may effect the system response time much more.
Because the amount of RAM and CPU consumed doesn't always give you an accurate reading if the product will run light or heavy. It all depends on how your system handles the resource usage.
One example of this: on my Windows 8 laptop- Firefox with several add-ons runs lighter than the rest of the browsers that I have tried, even though it still uses more resources than IE10 and Google Chrome.
The only way to know how a product will run on your system is to give it a trial run long enough to tell. Ignore RAM and CPU usage on Task Manager, when you launch a program, Windows Explorer, browsers, etc. pay attention to how long it takes them to start. While they are running notice any lags. If you notice system response time is longer than what it was on your previously install product then you will know that product runs heavier on your system. If the system response time is shorter than your previously installed product then you will know that products runs lighter on your system.
It is best to make sure the products updates are all installed and system has been rebooted several times before testing the response time because some security product take longer to completely setup, some will run scans of your system after installing which will make your system response time much slower. You will not get an accurate reading of response time if your installed product is running scans, updates or system checks.
Enjoy!!