I bought a SanDisk Ultra II 16GB/15MB SDHC Card to use in my Sony XDCAM-EX1 camera and it works perfectly well with the highest bit rates (1080p30HQ at 35 Mb/s bit rate).
However, the card shows that the max bit rate is 15 Mb/s.
So can you explain what it means ? Why should a card showing a linit of 15 Mb/s works well to record clips at 35 Mb/s rate ?

Look carefully at the capitalizations used:

Mb = megabit
MB = megabyte

Ignoring the overhead involved in various types of data transfer, there are eight bits per byte.

The codec rate is 35 Mb/sec (which somewhere between 4.5 and 5.5 megabytes per second)

As for speed ratings on flash cards, they are usually fibs told by over-zealous product marketing drones.

BTW on the back of the package of the Sandisk Ultra 2 32GB cards it says 9MB write 15MB read.

I did a read/write test Crystal Disk Mark and I thought the results were 20MB write 17MB read or something like that.