Why do 4s electrons fill before 3d AND are lost before 3d?

[blackhole123]

I can accept the explanation for both phenomena independently. 4s is lower in energy than 3d so it fills first. And 3d penetrates better than 4s so 4s leaves first. Accepting them at the same time does not seem possible.

 

If 4s is lower in energy then how can 3d penetrate better? If one is true then it seems to make the other not make sense! If electron filling is based on energy levels, then how can something be lower than another shell when filling, but higher in energy when losing?

[ianhowson]

I also would like to know.

Does nobody know?