Measurement of Crochet Hooks?

I'm a little confused with crochet hook sizes. Recently I bought two steel hooks: the first one is labeled IMIA 1 (1.60mm), the other JMIA 5 the mm range wasn't included. And my old crochet hook I use for making doilies is IMIA 7 (0.95mm). I already looked it up in the available crochet hooks conversion charts in the internet and it seems those don't tally with the measurements of the hooks I have. So if anyone could please enlighten me with the confusing variations in crochet hook sizes?