Factory Made PCs and Cooling

How is it that my sisters amd 1.4 compaq runs fine without overheating but mine that I built (also amd 1.4) had to get fans, new heatsink, then modded case side so more fans could fit? I dont understand why her computer which has all componets squezzed togeather so there is no room for air to flow didn't lock up like my computer used to before I added many case fans. I know compaq has engineers working on that kind of stuff but it doesn't make sense any way I look at it. I had more airflow but still higher temps before I made my computer a 50 decibal ear-killer while hers runs at a wisper because it has 2 fans (including the psu fan!)