You say "potato", I say "starchy tuber"....

In the comments to this post, Sean corrects my spelling of the word "estimable", which I had spelled "esteemable". Looking it up, I see that he is, of course, entirely correct; but this word is just bugging me now. I'm a good speller (although in the heat of the moment I'm likely to use "there" instead of "their" or "hear" instead of "here"), and I've long known that the English language's spelling conventions tend to not make a lot of sense, to put it mildly. This one, though, is sticking in my craw. If we have "esteem" and "esteemed", why on earth do we have "estimable" instead of "esteemable"?

Up with this I will not put.