What is the difference in meaning between the eastern European surname endings 'ski' and 'ovski' and 'ic' and ovic'?

Names ending with -ic or -ovic may be mostly found in Russian and are called "patronymics", which means they are derived from the name of the person´s father, take for example Alexei Sergeyevic Pushkin - his father´s name was Sergei.

There are two more common endings in the last name, -ov and -in, both answering the question "whose?" -ov is derived from a masculine noun (Volkov - belonging to a wulf, Petrov - belonging to a Peter), -in from a feminine noun, a nice example being Lenin who named himself by the river Lena - then, belonging to Lena)

Names ending with -ski are adjectives and mean "belonging to" or "coming from" like Kowalski is someone who comes from a smith family (kowal is a smith in Polish) or Warszawski could be someone coming from Warszawa.

https://en.wikipedia.org/wiki/Slavic_name_suffix

That's separated the two parts which is most of the job, thank you. Now I wonder what the pair stuck together signifies, as in for example Jablonski (now know means someone from an orchard/apple trees) and Jablonovski which should technically mean something else as there are many of both in Poland.

The difference isn´t big. Jablonski is derived directly from "jablon" (apple tree), while Jablonowsi is derived from Jablonski - someone belonging to Jablonski family.

There is another way - Jablonowski can be derived from a place called Jablonov, someone coming from Jablonov. A lot of Slavic places end with -ov, meaning the same as English 's, like in at the grandmother's (place). A forefather must have been called Jablon and his place, castle, or a village was then Jablonov. If the person was called Jablona, which is a feminine noun (but the bearer could be a man), the place would be called Jablonin and a man coming from there would be (hypothetically) Jabloninski.