Why do icons for disks differ from icons for logical drives?

You are seeing different icons in Disk Drill’s disk list because the hardware disks and logical drives (or volumes) are different entities by nature.

Physical/hardware disks correspond to real hardware in your computer and have sectors, tracks, cylinders, etc. Physical disks can be divided into logical drives and volumes in which users can directly perform file operations. Sometimes a logical drive may be spread over several hardware disks, hence Fusion Drive. The purpose of logical drives is to provide a contiguous storage area and free users from dealing with complicated operations on multiple physical units.

In Disk Drill, physical storage devices are denoted with icons showing an HDD or memory card, while logical drives have black square icons with OS logos (Windows or iOS, or other). This generally helps distinguish real storage devices from partitions. In macOS most logical volumes won’t have a dedicated system icon.