This is because for a fair comparison you need to scale the L1/L2 cache size as well. More cores - more cache. It is also important to not scale below 2 cores,

This is because for a fair comparison you need to scale the L1/L2 cache size as well. More cores - more cache. It is also important to not scale below 2 cores, because servicing interrupts causes performance hiccups. It is enough to have the second core heavily downclocked, and you'll still have better performance then with just a single core.