Why does a Quad core not perform better then a duo core?

My Dell E6400 laptop is still working perfectly now with a Crucial 512 GIG SSD and a harddisk cradle  with a hybrid 512 GIG harddisk. With 4 gig of ddr2 memory this dual core machine is as fast as a quad core HP 6550 with ddr3 and the same disk setup crucial + hybrid.
A Windows 10 update started at the same time and when the HP is on 95% the Dell is on 90%. I would expect a quad core and ddr3 to outperform the E6400, but is does not.
Is Windows update just using 2 cores on both machines or what else could cause this small difference?