Yes you're absolutely right, offsetting isn't perfect. There are pros and cons to offsetting your flight - it certainly shouldn't be considered the answer to flight emissions - it's not.
However, if you are flying, offsetting is certainly better than nothing. It goes some way to minimising the impact of your flight, and by double offsetting it further reduces the harm caused.
We've written a short guide to offsetting here34, which goes in to a little more detail. Have a read and let us know if you have any further questions.