It's not scrubbed from memory on short term sleep so the OS can provide a more user friendly experience – rather than dropping you at the EFI login window when

It's not scrubbed from memory on short term sleep so the OS can provide a more user friendly experience – rather than dropping you at the EFI login window when you wake from sleep, you'll be presented with the more capable and better looking OS sleep unlock window. If the machine is asleep for "a while" then it will write out a sleep image file and power off RAM (and other hardware) to go into a deep sleep. Since the sleep image file is written to the FDE volume, you must first unlock the volume at EFI's login window to gain access to the sleep image and resume from sleep. That process takes far longer than waking from a warm OS, so it's not done by default. You can change sleep settings using pmset on the command line and force it to always destroy the FV keys on sleep, if security is more important that a quick wake from sleep.