Letters in Greek alphabet are ordered similarly to those in Latin, but not exactly the same. Is there any reason for the differences? I ask the question for finding a way to understand and therefore memorize the order based on Latin alphabet.
There were many versions of Greek alphabet. Italic scripts were based on Western Greek alphabet, while later standardized (by Eucleides) classical Greek alphabet was based on Eastern variant of Greek alphabet.
E.g. When Zeta letter (Ζ, ζ) was originally invented, it probably meant [d͡z] affricate, which later changed to [zd] 2-consonants cluster in some dialects.
For the most part, the ordering is the same, inherited from Phoenician. But some letters are used for different purposes in the different alphabets due to different transmission routes (eta vs H, gamma vs C), and others have been lost in one or the other (Greek lost F, Latin lost zeta).
Both of them also introduced new letters over time, and put them at the end: Latin borrowed Y and Z from Greek, for example, while Greek invented psi and omega.