[Yaffs] YAFFS vs JFFS2

Hi,

I've done this writing on NAND:
1) 4000 files whit size 4k
2) 250 files, 64k
3) 32 files, 1M
4) 4 files, 8M
5) 2 files, 16M
6) 1 file with size 1M.

And, the results are:
      JFFS2        YAFFS
      -----        -----
1)    1m50         2m1
2)    0m15         0m42
3)    0m17         1m18
4)    0m17         1m21
5)    0m17         1m24
6)    0m25         1m28

The results are much differents. Why?

Thanks.

PS: I've a bast machine, with a NAND STMicroelectronics and
a NAND Samsung.