micro SD speed

[Tassadar]

jjwerner@localhost:~$ dd if=/dev/zero of=test oflag=direct bs=8M count=64
dd if=/dev/zero of=test oflag=direct bs=8M count=6464+0 records in
536870912 bytes (537 MB, 512 MiB) copied, 24.029 s, 22.3 MB/s
jjwerner@localhost:~$ dd if=/dev/zero of=test oflag=direct bs=8M count=64
536870912 bytes (537 MB, 512 MiB) copied, 24.0569 s, 22.3 MB/s

Sandisk Extreme 32GB U1   micro SD card

Good card, glad to know that read speed can achieve 22.3MB/sec.

Should this micro sd read at least twice of the speed, shouldn't it?!

Did i missed something?