There is no real difference between the two. One is a passive verb while the other is an "active".
Pe-Nun = po nitman  - translates to here was buried (literally hidden)
Pe-Tet = po tamun - translates to here is buried (literally hidden)

probably just the style of the writer. The prevalent usage is Pe-Nun.

Yeshaye Goldberg - researching Limanov, Krushnik, Zabno, Radlov. Families: Goldberg, Steger, Kellerstein