I run 3 projects on my 4 core PC, one-third to each. So I would expect the average credit to be the same. Yet this is what the BOINC client says:

SETI@Home 447.46
malariacontrol.net 429.38
World Community Grid 538.92

Why is WCG figure so much higher than the other two ?

The BOINCstats figures however are much closer.

Is there a bug in the BOINC client (I use 6.3.36), or is there another explanation ?

Many thanks.

There is no bug in your client in relation to the granted credit.

Your PC will claim the same amount of credit for the same runtime on each project depending on the benchmark of your PC.
Some projects (like AQUA) do not use the benchmark, but have a build-in-claim in the application.
This benchmark you can find in your Boinc Manager message tab after running benchmark (automatically done every 5 days) or on the host details on the project site.

This details cannot been seen on WCG, because they did not fully adopted the BOINC filosofy.
Some projects give the credit (granted) you claim, other projects give credit depending on their own rules or give fixed credits per task.
That's why there is a difference in your credits in spite of your equal share.
Another point is that you could have claimed but not granted yet. You have then pending results waiting on a result of a wingman.

Many thanks for explaining that !