I-84's "split personality" is unique in the interstate system and has nothing to do with the east/west-north/south numbering system.
The western segment began life as "I-80N", but somewhere along the line some folks with nothing better to do than mess with our heads decided there was something wrong with that designation/enumeration.
After much deliberation apparently involving a truckload of Jim Beam, it was decided that I-80N would be renamed "I-84" despite the fact that a road with that name/number already existed back East or, as known here in the Pacific Northwest, "nowheresville".
Note:
Some Historians believe that the Commission in Charge of Doing Stupid Useless Things was simply unaware of that fact. Others say they did know, but thought it a Great Joke to go ahead and use it again anyway.
See
this link.