looked up the -ible/-able thing in Fowler today. There's quite a lot on the subject. A very long article, and I'm happy to report that there are quite a few useful rules governing the usage.
I have one question:
why can't I spell?
When I was in the fifth grade, I was tested at a tenth grade reading level. I started learning about writing, style, grammar, plot and character development, in the early grades in school. I was working and being coached on developing a salable style when I was eight. I've been writing for a living full time since 1999, a good bit of it for an audience of people in government.
In all that, I STILL CAN'T SPELL!
I suppose you've noticed.