I imagine this is complicated by the tendency to abbreviate numbers with the Greek numeral system, as opposed to writing them out the way they would have been pronounced.
If I had to guess, it seems to be a trajectory between Classical teen numbers that end in δέκα systematically, vs later Hellenistic variants that begin with δέκα. At the very least, I know the Modern Greek variants all ended up settling on a fronted δέκα from 13 through 19. So I'm just curious how exactly it got to that point over time.
Here's a list of variants I've come across in the neuter:
- 11. ἕνδεκα
12. δώδεκα / δυώδεκα / δεκαδύο
13. τριακαίδεκα / δεκατρία
14. τεσσαρακαίδεκα / δεκατέσσαρα (alt. -ττ-)
15. πεντεκαίδεκα / δεκαπέντε
16. ἐκκαίδεκα / δεκαέξ / δέκα ἕξ
17. ἑπτακαίδεκα / δεκαεπτά
18. ὀκτωκαίδεκα / δέκα καὶ ὀκτώ / δεκαοκτώ
19. ἐννεακαίδεκα
And for "19" it doesn't seem like Modern Greek δεκαεννέα (Demotic - δεκαεννιά) is attested at anywhere in Ancient Greek sources. I guess people held on longer to the Classical form of that number?