Hard drives just about always have caches, typically 8 GB but often much larger.

They have both read-ahead and write-behind caches. The read-ahead will read the next few blocks after a a block was just read in anticipation of them being read in the near future, but will remove them from the cache if they don't get read before the cache fills up with other block.

Your SDHC card probably has no cache at all. While it would benefit from the fast seek times, you would be seeing the direct read and write behavior of the flash.

Writing flash is very slow because it must be erased and then written to in a second pass.

Flash wears out when you write to it too many times. Once you've done so it won't store data reliably. To prolong the life, wear leveling is used in which the physical sector used to store a logical sector number constantly changes. Thus there is effectively a filesystem underneath the filesystem that Linux actually sees.

The wear leveling is implemented by the hardware of your SDHC card and may not be implemented with very fast circuitry.

Flash can be made very, very fast as with solid state drives with SATA or SAS (Serial Attached SCSI) interfaces, but I expect those drives do have large RAM caches as well as faster wear leveling hardware.