Hi Kyriakos,
If you look at the block documentation, you will see a diagram of how the "Big Endian" (BE) format fills up the bytes.
If you start at bit 15 in BE format, then the bits will be filled in the order
15 0 1 2 3 4 5 6 7
... which means that starting at bit 15 only gives you up to data length 9.
If you want to throw a 16-bit value in the first 2 bytes with BE format, you need to start at bit 8 such that the order is
8 9 10 11 12 13 14 15 0 1 2 3 4 5 6 7
Try it out!
- Sebastian
