Emmanuel Gustin wrote:

According to McBean & Hogden in "Bombs Away", the bombs
used in the Amiens prison raid were 500lb MC bombs, fitted
with 11 second delay fuses; dropped at very low level to skip
over the ground and hit the prison walls. The walls were
reportedly about 1m thick, hence the bombs would not easily
go through them. I suppose fairly long delays were used to
protect the bombers from the blast of their own bombs.

This is speculation, but

If skip-bombing over the ground, wouldn't the fuses start to run on the
first impact? Which would be pretty much directly below the attacking
aircraft? Eleven seconds may have been chosen to allow safe separation
for the worst case (bomb dropped early/bomb missing the walls and
following the aircraft...)