I will write my Indo-european and Greek in Latin alphabet, cause I use other Greek fonds than you do. I have to check the post about it! O my!
Anyway the Latin alphabet is easier for indo-european also.
You all know that Greek is a indo-european language, as Latin, German, French, etc. All these languages have similarities, so scholars found out that they all came from one language, Indo-European (sometimes called Indo-German(ic)). This is one issue, that we can consider to come to Greek forms.
In Greek we have also the dialects.
First we have to know that some Greek words with strong similarity, come from the same origine, but that they evolved differently.
Here with Thalassa -Thalatta there was a different evolution in the dialects.
We know that there was a letter (dissapeared yet in ancient Greek) named jod(h) and it sounded like j / y. This letter jod dissapeared in ancient Greek, but is still important to see some evolutions in Greek.
When an occlusive was followed by a jod(h) in a word (!) they evolved to double sigma in the most dialects, but in some they evolved to double tau.
E. g.: glôch -ja > glôssa, but in Attic glôtta (for glôch -ja: see the Greek glôchis, ...);
peku -jo > pek -jo > pessô, but in Attic pettô, etc.
Attic had double tau from the earliest times, Ionic had double sigma. In the later koine double sigma is more common than double tau. Most of the dialects agree with Ionic, but double tau, as in Attic, is also Boeotian, Cretan and Euboean, at least in Styra, Eretria, Oropus (as we find in inscriptions). In Cretan, we even find sometimes double theta!
Why this evolution was different in the dialects we know, is always a guess. I suppose there are some theories, but these are still guesses. And I think we have to confess that some things will always be a mystery for us. Asking why the evolution is different in different places is the same as asking why there are dialects and why not one universal language. But that's an other question.
I can conclude with saying it's a difference between dialects.
I hope I was a bit helpfull,