No you are not! There are rules dictating those spelling differences but they are certainly not obvious and there are exceptions (no surprise there!). It has to do with word derivation: what is the verb/adjective ending from which the noun is derived, where does the stress go in the word, does the noun show an action or something else?
So αληθεύω gives αλήθεια but επιθυμώ gives επιθυμία. I have to admit it is very easy to forget what the rules are, or that they exist at all, as they are definitely taught in (primary) school but what happens a lot once you know the language is you figure them out from the spelling. :)
A quick search for "λέξεις σε εια" or even "θηλυκά ουσιαστικά σε ια και εια ασκήσεις" actually returns many results that basically say the same thing. All the following links are therefore in Greek, but I think you can use them: rules - link 1, rules - link 2, see exceptions and practice here, more exceptions - practice - 1, more exceptions - practice - 2.
Many thanks D, yes it is quite complicated but the basic rules that you cited are helpful. I found the last book "Παιδίου Τόπος" the most difficult to follow, basing the rules on where the accent falls is problematic for non-Greek speakers. Incidentally what does " καταχρηστικό διφθόγγο" mean? With gratitude for your help.
Yes, in the last link just note the exceptions of common words like δουλειά, χημεία, φτώχεια.
Καταχρηστικός δίφθογγος is a false or spurious diphthong (two vowels considered as one when you think they shouldn't/couldn't/wouldn't!); see previous comments here and here. I have no idea how to explain the phenomenon, it just happens and it is understandably problematic for non native speakers. I'm glad you're looking up grammar in so much depth, shows you're really dedicated to your studying! :)