Why E-boats penetrated a D-Day exercise

How did E-boats get into a D-Day practice area despite the expected defenses?

The defenses were constrained by ships being withdrawn for maintenance, radar and gun upgrades, and final preparation for the invasion, while human mistakes also left gaps.

Britain was trying to defend the rehearsals, but the available force was not identical to the fleet assembled for the invasion itself. Ships were undergoing maintenance, receiving radar and improved weapons, and being readied for the maximal effort ahead. Those necessary preparations reduced immediate numbers and created openings at the exercise stage. Human failures in a complex defensive chain also mattered alongside the material shortage.