SSD performance different results in dd and gnome-disk

I wanted to measure my SSD read/write performance and found some recommendations for using dd and gnome-disks. (from my understanding, hdpart is not relevant since it's not using the disk itself but only cached data). The problem is that I get different results with these tools and I wish to understand why it happens. The rootfs is mounted from a local internal eMMC and the SSD has a mounted ext4 filesystem. Thanks