Thanks for the answer.
I don't know why, I cannot post the link, but a quick search of Avoid Misaligned Vertex Data Apple, shows that at least some major Apple devices are doing attribute padding on...

I have a compact interleaved vertex format to draw simple objects :

2 bytes x 3 : position x/y/z
2 bytes x 3 : texture u/v/w (w is layer num in texture array)
3 bytes : normal x/y/z
1...