Why build's user CPU on 4-CPU machine with hyper-threading always higher with -j 8 compared to with -j 4?

[Yuri]

I noticed that the same exact build on i7-920 (4 CPUs) consumes ~15% 
more user CPU when run with -j 8 compared to -j 4.

Hyper-threading is enabled so top shows 8 CPUs.

Why would user time be higher in a  hyper-threaded run?

Yuri