I hope I can explain this mathematically for you, if it helps.

The equation of approximate probability of have at least one successful melding attempt with a 7% chance is equal to 1 - (1-p)^x, where 1-p is your odds of failure and x is your number of attempts.
Let's ask ourselves how many times must we attempt a meld in order to guarantee that one successful overmeld ? Our equation becomes 1 - (1 - .07)^x = .99. Now we need to find x, or the number of times needed to meld to guarantee a 99% chance of an overmeld within x number of tries. Reorder our equation to get .01 = .93^x reorder again to x = ln(.01)/ln(.93).

x = 63.56.

This means that you have a 99.00% chance of a successful overmeld within ~64 tries.

Now you said you tried 160 times. You had a chance of at least 1 chance of a successful overmeld of 99.99%, but not quite 100% chance. And as we know, if it's not 100%, then nothing is guaranteed in the world of crafting.