Why is the CPU's behavior different with same test suite?

I ran a same test suite on two different platforms: 
1) Drive server : redhat5.4, under test server : aix6.1
2) Driver server: redhat6.0, under test server: win2k8
There is a CPU usage increasing trend on #1 drive server as the chart: https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhRC1SxPskw7oENwotLAxbfza22irk26H2y06wjyIu_lOT5Bzqxey9GIabwoDCU3WMENUXRT5F1YAtM1XvT6Wygki-ehkgaJH5GLd6ZZ2U0MF17EArlE8yzdoANUL9tW4HwsxjNrEkZNU04/s320/cpuIncreasing.PNG, but it is really stable on #2 drive server as this chart .https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjhflIFcJ2TV7sRGTJAOICbwRpS9tWXvfQhb85Qe43RLVOA_rFafbLi_DFlWEZIfkIcXZWiWoEbePbm2cgg4Ezv3BIdm2G5lC91mHqwqrDFRSo-gY2HtNfG9htl3RA-v5k3q8epAdelngsm/s320/cpuNormal.PNG I checked the network transfer rate is also increasing on #1 drive server as the chart https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg65JEwv-Z6Y-OrxTILGwajh3uirIVrA2QAnQu8nEOC2jnkL-nsshKR-cTA-y8oCahm78Q0ee1t5Xk1lFBenHVxtKzRVkU8Gi-zXjB0_Gf0eqEaQ5eGcIGkzUZmioWbBge4C9vof09oDEgf/s320/eth0_netpacket_read.PNG so I think it maybe the reason which caused CPU increasing, but I am curious why it didn't appear on #2 drive server?