Depending on how you define the words it could be considered physiological and genetic. The cells have to actually fuse together and even have limited genetic transfer at the graft location. People frequently try to graft unrelated plants and sometimes succeed. Grafts between same-species plants is often possible. Grafts between different genera in the same genus is not uncommon. Grafts of plants from different genera are somewhat uncommon, but have been successful in some cases.

I think the inquiry would be fine for the proper venue at school, but I'd probably ask something more akin to, "Can you explain what specifically cases some plants to be able to be grafted together but some not able to be successfully joined?" You might also ask about why some types of grafts can be successful initially, but then fail later on.

It would be cool if you came back and discussed the answers you got, if you decide to ask.