Do you still have the little booklet that came with the extension board? I ran into the exact same problem with this kit.
I had the same problem with P7= GPIO 7 (BCM 4) but P6 = GPIO 22 (BCM 25). If you have the booklet, the table on page 5 has a "Header" column in the middle with the correct GPIO pin numbers. This table is laid out to match the pins on the pi, not on the extension board. To use it, you'll need to look at the wiringPi values and match those with the diagram of the extension board or the labels printed on the extension board to get the physical location of the pins. At some point, I'll print a table that is laid out to match the extension board and includes the "header" values.
The whole booklet is pretty bad. I'm glad I didn't damage anything while figuring this out. If they just labeled the extension board with either the pin name or the "header" number instead of that random wiringPi number, it would be a lot easier. I'm pretty new to this, so maybe there's some use in that number that I'm unaware of.